Why tell me that you knew dingdangity well that you were taking a photo that wasn't yours? Why not leave it to DH to explain to me what happened to the photo?
Your DH let her have the photo. If he didn't want her to have it, HE should've spoken up at the time. He's letting this happen - he's got to be on board with change before it's going to have any effect on your MIL.
This. MIL knew it was perfectly possible to have another copy made for her OR for you. Since she wanted a copy why not take that one instead of you having to bring a different one by? MIL had no idea that the frame was part of a set. During the exchange only your DH knew that her taking THAT copy would impact your plans.
To me, this is a lot more on your DH than your MIL.
I agree. Why didn't your DH tell his mother that the photo wasn't for her, but he could get another printed off for her, if she liked? Is he that much under her thumb? Or did he simply think it was no big deal to let her keep it?
Anyway, I like Toots' suggestion for dealing with the situation, given that you chose and paid for the frame.