I have no problem with MIL's seeing rings
Taking a photo is a bit weird, but OK that might happen.
Taking a photo and then OOPS, mistakenly showing your future DIL, "silly me", with who you don't have a fantastic relationship is hugely dodgy
Just what Panda has said.
He may have bought the ring and shown her because he is so excited. Or maybe she "accidentally" came across it while she was raking through his private belongings to feed her paranoia and find something she could manipulate you both with dusting.
Taking a photograph of it is distinctly odd unless she was showing her friends "Look what he's bought that little cow and all I got for my birthday is batch salts-- "
"Accidentally" (she seems to have a lot of "accidents") showing you is definitely shit-stirring. She WILL drop you in it if you don't mention that you've seen it.
There have been some good suggestions as yo how to let I'm know that you know - or you could say, "Please don't be annoyed at your mother but . . ."
It's a difficult situation, because as others have mentioned she could be winding you up and downloaded a photo from online, or take a picture of a friend's ring or something (I'm not suggesting your DP doesn't love you, just that he might not have this in mind just at the moment.). She might be hoping that you'll scare him off if you seem too excited, and she has an ace up her sleeve if you don't mention it.