I wouldn't worry about it. It was just a picture of two friends. I'm sure someone could find pictures of you and old boyfriends from long ago. If you're that upset by it, go to him, not his best friend and start out by saying how insecure you're feeling and now you just need to be assured that Sue (or whatever her name is) is simply "just a friend".

If you continue to have these feelings then you'll surely sabotage the best thing you've ever had. It can become a self-fulfilling prophecy.