OP - I am having a change of heart about this.
I've been rereading, and this has been going on for weeks according to your first post. How many weeks?
How long has it been since the wedding and how long after the wedding did you send the photo. How long has it been since your MIL replied?
Because I was thinking it was only a couple of weeks. Now it sounds like it has been a few more than that.
I still think they have a right to feel hurt and excluded, and to some extent they have a right to try and explain to you how and why they feel the way they do.
But my in-laws are also the type to cry for weeks, to threaten to end relationships, to blackmail and manipulate. It was only last year, in the week between Christmas and New Year, that they disowned my DH because we went for tea at his sisters house and they felt we had snubbed them by spending time with her instead of them. They told DH he was a "tiny part of their lives that was over now" and threw various insults at us along the lines of us thinking we were better than them, that we are liars and that we take advantage of them.
My MIL also tells everyone she knows about how hurt and offended she is and how she is blameless in the rift that has occurred between us (not so).
So I know how exhausting it can be when someone drags something out for weeks or months, when it could have been resolved in a much shorter time.
You are right. There is a big difference between saying something like "you look so happy in the photo's, I wish I had seen you" and "I'm so upset because you look so happy and we were not invited!" or whatever it was she said.
But you have wrong-footed yourself now. You might be absolutely right about their reaction if you had told them beforehand. The fact that you didn't has given them the chance to deny that and put all the blame on the two of you for never giving them the chance to react any other way.
My MIL has denied her wrongdoing so often I think she has convinced herself she is telling the truth and that it is all me, even though her behaviour has been directed at my DH too, and we have witnesses to some of it. She could pass one of Jeremy Vile's lie detectors now, so convinced has she become from her own lies. Your MiL may be doing the same thing ("We would never have forced you/pressured/demanded anything..").