I don’t think I can offer you sensible advice without leading the speculation into five different directions… but let me try.
Firstly, he may be overreacting, but on the other hand, he has some “rights”. Those past boyfriends don’t have any. Therefore, if it causes him anguish that such pictures are brought up in his presence, I think his feelings should be respected.
If he, however, has a problem with the very existence of such pictures, would wish them burnt or something like that, then he’s probably strongly overreacting.
At any rate, reminiscing about past boyfriends serves no good of itself. If it’s regarded as a picture of the lady and the past boyfriend just happens to be in it, then it’s one thing. But if it’s specifically treated as a picture of the lady and her past boyfriend, then it’s essentially reminiscing about the past boyfriends, which might bring some nostalgy and a pleasant memory of auld syne and whatnot, but it doesn’t trump a husband’s feelings. Even if it doesn’t immediately touch his rights, even if the feelings are somewhat unreasonable, those human insecurities should inspire some leniency and the feelings of such close people should be respected. After all, he’s the husband and the in-law. Those guys are not.