DH and I take a lot of pictures. When we post them online, we always make sure that there are no bad pictures of people - no pictures with funny expressions, no blurry pics, no shots of people eating, etc. There are times when it turns out that we end up with no pictures of some people at a party because posting bad/embarassing pictures isn't something we do.
Now, we haven't seen the photo set in the OP, but it's entirely possible that there were no good pictures of MIL for DIL to post. So, what was she to do? Post a bad picture for the sake of inclusion and risk a snarky response from her MIL? Or post no pics and get a snarky comment from her MIL? There's no winning here.
I am the same way with pictures, I may take a 200 picture and only have 70 worth keeping.
I had a difficult relationship
with my MIL. She told family members that she hated me, and was critical to my face, nothing I did was good enough. It got to a point that everything she said to me seemed critical, I know that she may not have meant things to sound critical, but that is what I heard.
It is possible that the DIL left out all the pictures of her MIL, or had no good ones to post, or just didn't take that many of MIL.
MIL is hurt, but tries to say something nice and the hurt shows through or their relationship
was so bad that the DIL couldn't see it as a nice statement.
Then DIL goes to her husband, gives an off hand remark about his mom being snarky again and he talks to his mom hoping to fix things or DIL demands that he talk to his mom about her snarky behaviour and the husband talks to his mom hoping to fix things.
All of these show a difficult relationship
that does not rest on one comment.