Ideas
I was afraid the cause might be something like Down-in-Simsland says. They have the vocabulary to describe it far better than I do, but reading that has at least validated my decision to not open any photo-heavy saves since I first noticed the issue.
I know this might not seem like a big deal at first but this is really, really bad for legacy players. I take great pride in the pictures I take, knowing that I'll be able to hang them up and look at them months later when the Sim is old or gone. What's the point of doing a legacy if you can no longer properly remember previous generations through photos? I have 22 generations worth of pictures in one of my saves, and have spent literal days framing and timing them so I can remember them how I want to. I'd be okay deleting a lot of them if it meant curtailing the save corruption glitch, but I certainly wouldn't have chosen the large photos, which I made large because they were important and now are so blurry they're hard to look at.