I have bit of a different take, I don't think "excluded" from pictures is the right word or phrase, because MIL was not. She was not asked to step out of the pictures or deliberately not taken a photo of, the photos just were not posted to her DILs facebook page.
Since there were 20 people there, it seems like looking to make sure the other 19 were represented in DILs pictures seems like a very intense interest and somewhat obsessive. It would be different had the MIL said "I wasn't in any of the posted pictures" to the OP, but what it is coming across is saying that she was *excluded* therefore she went and looked through 70 pictures to make sure that the other 19 participants were in someway represented. That's a LOT of time and effort for something that is on anothers page.
On another note, she could have just asked for some pictures of her if they meant that much, she really didn't have to comb through her DILs pictures to make sure 19 other people were in them and then exclaim she was *excluded* which makes it out to be a bigger deal than it is, the only thing that she did not get was pictures of herself on her DILs facebook page. Her behavior to this as described is seriously overkill in my books and I can see myself saying the same thing as the DIL here to my husband, it is snarky and very passive aggressive. If you are that upset, you could contact your son to get the pictures. It's clear from what she told the OP was not about getting pictures but about being represented on her DILs facebook page, that is very controlling to me.