I think the problem is in your last paragraph. You have issues and so the presence of photos ticks you off and makes you feel insecure, AND he isn't open and honest with you but instead hides things.
So the photos are not the problem. The lack of trust and other underlying issues are the problem.
I talk to the guy who took me to the big dance in high school, and we saw each other occasionally when he lived closer to me. And my husband has lunch a couple of times a year with a woman he dated in high school. Neither one of us has any problem with this because we have complete trust, we tell each other what's going on, and we're thrilled that the other has friendships where old times can be laughed about. However, neither of us has ever even been close to a moment of scandal or cheating or indiscretion.
You admit you are not feeling very secure, and it's not clear to me if your husband feels he's "protecting" you by not disclosing more, or if he's hiding something. Sounds like this woman came out of nowhere and friended him, then posted an old picture. If she doesn't have designs on him, great. If she does, they you and he together have to discuss it and see what the best way is to handle it. Maybe he should un-friend her if she's going to be a problem. But if this is just an on-line high school reunion, and if both your husband and she are happy in their separate lives, it's not a problem. If he's saying to her, "Oh, my wife and I looked at this photo and neither of us could believe how young I looked then" or "I'm not showing this to the baby or I'll scare her to death" then that's great. It lets her (and the world) know where his priorities are. If he's not talking about you or the baby on FB, and again, if he's not showing you things, that is a bigger issue.
I'd look at the deeper problems that make this photo a problem, not at the photo itself.