Anyone would feel left out in your situation. I don't really have any advice, but i sympathize. 

Like Rumplestiltskin says, anyone would feel left out, and your wife in my view, has behaved extremely insensitively in not including you in at least one of the photos.  I have been excluded from things involving my (now) adult stepdaughters more times than I can count and it hurts.  Don't let your wife DARVO - ie deny and reverse victim and offender.  She should damn well feel guilty!  

I'm sorry that the situation hurt your feelings.  Sounds like they also were looking for keepsakes for their own homes.. and unfortunately.. despite your long tenure in their lives.. they may still. see you as "mom's husband" vs a parental figure.. 

If the bond hasn't been forged by now.. it likely won't be further.

It sounds like you were included in the largest family picture.. so while not heavily featured (in the photos they want in their homes).. you will have a couple at least where you are included.. I would feature those.. the others can be in a book where your wife can admire them.. withoiut you having to face them.

It goes without saying.. that you would not need to bend over backwards at this point financially for her kids.. if she wants to fine.. you have your own 5 you can dote on that way.
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You're not overreacting and you have a right to feel hurt. Being purposely left out of photos is like someone giving you the finger. Your wife should not be ok with this nor should she encourage it or go along with it. But if this behavior has been tolerated for 18 years, how do you now turn it around? 

That large photo with out me would never hang in my home.  This is a DW problem .. She did not include you.  She was there when the photos were. Being there and did not say.  STOP. we need DH   This was totally disrespectful,  unfortunately you allowed this disrecept to happen for 18 years.  As no fathers day cards.  That is disrespectful,  

I've been at this step-thing for 20 years now. Bottom line is that if the skids haven't accepted or appreciated you now, they never will.  Hard to hear but it's the truth.  And unfortunately, this is true in the majority of stepkid situations. 

My best advice is to disengage as much as possible. While it hurts to think you've wasted so many years on people who have shown you nothing but ingratitude, now you know the truth - the photographic truth. As the adage says, "When someone shows you who they are believe them." These kids (adults, actually) have shown you that you matter little, they don't appreciate what you do for them, and they don't respect the fact you are their mother's HUSBAND of almost two decades. 

The change that needs to happen is your mindset, which takes practice and is part of the disengagement process. Beginning today, admit to yourself these kids are not going to come around and your DW is not willing or able to influence that in any way.  There is a section here on StepTalk which is "Disengagement" - suggest you start by reading up there. 

I know it's hard after 18 years of trying to suddenly drop the rope but it is possible. My advice to people is DO NOT announce to your DW that you are disengaging from her kids. Start slow if you must. When the skids come around to visit, find a reason to be gone from the house or go do a project/activity away from them - tinker in the garage, go to your bedroom and watch a movie, etc. Don't say anything more to them than a polite, "Hello" and mabye a cordial but superficial "How are you?" Then extricate yourself as soon as possible.  If your DW wants you to go visit them with her, find a reason to get out of it. Even if you have to feign illness, work overtime, volunteer somewhere, or say you need to go to help a friend with a project . 

Consider the skids like you would young coworkers of your DW - you know who they are, and DW needs to have a "working" relationship with them, but you do not.  Not your circus, not your monkeys. (To paraphrase another old adage.)

When your DW wants to have any kind of in-depth discussion about her kids find a way to extricate yourself from that, too. Start by saying non-sequitors like "Oh, that's interesting." or "Hmm, " and then change the subject as smoothly as you can. If you need to, get up and go to the bathroom just to get out of any conversation. You don't need to waste another minute of your life listening to issues related to people who don't give a damn about you.

While 18 years is a long time to have spent on these ingrates, consider this: YOU are the better person. YOU are the one who made a positive difference in their mother's life, and in their lives. Although you will never get credit for it, YOU know what you have accomplished even if no one else does.  You can be proud you took the higher ground.

Now you're work with them is done. Retire them from your life. 

Im in Camp Disengagement for how you are to deal with this subject going forward.

Ive got over 10 years in this...

I am no contact with SD (stepdaughter) 25 Feral Forger, and disengaged from SD(stepdaughter)18 Princess Powersulk Do Nada, our Lady of Intertia.

Anytime Ive been involved I typically get my head chopped off somehow. It has ALWAYS bitten me. Ive done a lot for SD18PPDNOLI in the past and now I do nothing for her except "oh thats good news..." or "hmmmm, wow, bummer". They both have moved out, thankfully.

Do not extend yourself for those who do not appreciate you. In fact, if you have any assets, make certain that your kids are covered. Focus all your love energy on your kids and make certain your wife doesnt hang her golden uterus products on your walls. Her turning it into a guilt trip shows you who she is. Stand firm.

Your wife should be your advocate. You are her HUSBAND, and if shes not going to correct them, after 18 years, you must do it. Next time you are referred to as her PARTNER, say something like "Oh, did DW divorce me without telling me?" haha.

Welcome to steptalk. The club no one really wants to be a member of.

I've been in the game for 28 years and am still not considered family. DH is far too yellow to correct the ingrates, so I do nothing. Just when I think I can't remove any more of myself I find a way.

Years ago when the mini-wife on steroids was a teenager, she and BM conspired to arrange for a professional photoshoot of SD, SS and DH leaving DD and I out. DD is also DH's bio. I constantly got gaslighted and told how wrong I was to feel the way I feel and I ended up with that photo at the end of the hall just outside DH's and my bedroom for seven torturous years before we moved. I told DH I was in charge of wall decor and you better believe that damn picture did not make it onto any of our walls. It's in the garage somewhere. It was all I could do to not smash the thing to smithereens so I wouldn't ever have to see SD's smirking, snotty face, but then I would be called evil, so I simply put it away where I'd never have to see it again and someone can have it should they ask. When we bought our house and moved in, I took charge of wall decor and DH knows damn well why. I even told him I will not ever tolerate evidence of how SD is the wife and I'm the inferior one in MY home ever again. 

Over the years I have removed my services more and more. This year I found yet another reason and another thing I do to be removed. I'm known for my Christmas baking and hosting. SS33 and SDIL30 have made no effort to have any holiday dinners with us since they met a decade ago. All the effort is for her parents or BM. Last year I put my foot down and told DH I will not hold Christmas dinner on a stupid day like the 19th or 29th to accommodate them and inconvenicence everyone else just because BM and the inlaws get all the efforts. Most of the time that we hold something they mysteriously come down with the flu only to post on social media how they're out yukking it up with friends, BM or the inlaws. Well, the day after Canadian Thanksgiving, I posted an event in the family group on FB. So, plenty of time for them to make arrangements. Well, of course they aren't coming. DH is doing his thing running around trying to arrange a casual cards exchange instead and you better believe there will only be store bought treats there because I'm not lifting a damn finger for those two. Fuck em! DH will have to clean the house, get the store bought crap and do the hosting because I'm nothing. Last time I did all the cooking and baking for Canadian Thanksgiving, SDIL sent only DH a message thanking him. Did DH correct her and say it was me and then include me in the response? Hell no! So, since the two ingrates can't even acknowledge me and my efforts after almost three damn decades and since DH has no balls to correct the ungreatful bitch who declares BM as "the best MIL ever," I'm done. I won't ever do a thing for those two assholes ever again. I also will sit in a corner and not say a word. Since I'm not allowed to speak of it, I find my passive aggressive ways to send a message loud and clear. Since I'm invisible I'll be invisible.

To be honest, I have an escape plan and every day I fantasize over implementing it. The bullshit never ends. Not a day goes by that I don't regret marrying a man with brats.

Is my tone bitter as hell? Oh yes! Yes, it is!

You and your DW entered into an equity life partnership. You have been all in. She apparently has not been in at all.

So, time to man up, set everyone straight.  Immediately inform your DW that from now on her ungrateful rude spawn will refer to you as Mr. LastName and not by your given name.  Inform your wife that no marital resources or time will be spent on them without prior discussion between the two of you.  Make sure that she has clarity that it is time for you and the marriage that you have together to be her priority.  As you have always made her and the marriage your priority

Stop being her beck and call boy  and the chore bitch for  her spawn.  You need to immediatley stop being the stuff they all scrape off of their shoes.

I have been in this game for more than 30 years though my SParent adventure is the Unicorn of SParent situations.  Yours is incredibly deep and complex.  My DW and I met when my SS was 15mos old and we married the week before he turned 2yo. My wife was one to give eternal second chances to the shallow and polluted end of my SS's gene pool  when they pulled their crap

She would stomp on their throats. They would crawl under their rock for a while, DW would decree that they had learned their lesson and back off. Immediately they would go back to their manipulative PASing lying bullshit.  After a few of those cycles I finally put my foot down and pointed out that they were taking their crap out on my SS anyway and the only way to protect him was to destroy them when they stepped out of line.  After that, she kept her boot on their throats and tolerated no crap.  We raised a wonderful man together.  Unfortunately we were not able to completely protect him from their toxic bullshit.  He does have some residual baggage from them.  A bio-dad who has never had time for him and who was focused on his failed gang-banger wannabe dreams, impregnating every underage womb he could pollute with his shallow and toxic gene pool, and avoiding paying CS.  My SS is the eldest of 4 all our of wedlock SpermIdiot spawned half sibs by three different baby mamas. The dipshitiot SpermDad never paid a cent of CS. The SpermGrandHag paid all of the CS for SS over the 16 years of the CO and the dipshitiot has lived rent free in a rental property owned by his parents for nearly 30 years.  I get having a SKid with a POS asshat as a SpermDad.

Eventually our son wrote them all off for being toxic and detrimental to his life and well being. 

SS is an only in our marriage.  He asked me to adopt him when he was 22. We made that happen. In his 30s he is a quality adult and a man of character, honor, and standing in his life, profession, and community.  He treats people well until they give him reason to not treat them well.  Then he gives them clarity.

I advise baring ass on all of them including presenting the facts about their POS father making it clear that you have been there for them for 18 years and he has not been there at all.  He was a fall down drunk at a wedding. You are not that guy. He is.

Stop taking a backseat in your own life and in your marriage.  Man up, make it clear to your DW and everyone else in the mix that they violate the standards of respect, behavior, and performance at their own risk and that you will be applying escalating appropriate consequences if they violate your boundaries.

Set boundaries, enforce those boundaries, and focus on living your best life in full view of all of them. If your bride refuses to listen enforce standards of reasonable behavior on her kidult children as treating her, her marriage, and her husband with respect, then she needs to feel the same consequences you bring to bear on your rude disrespectful SKidults.

Now for the most important part.  Welcome. I hope you find this to be a good place to vent, contribute, and to pick up some useful perspective from others who are living the adventure of the blended family dream.

Take care of you.

You matter, do not discount yourself.  Do not ever again sacrifice yourself on the altar of SParental martyrdom to your failed partner failed parent wife, her POS X, or their failed family progeny.

Live your best life.  Living well is what you owe yourself and your own children.  They do not need to see their father being the victim of his wife and her POS adult spawn.  Living well is also the best revenge. They can get on board or be left behind.