well JD i'm a guy and i had to read your post over twice so i could respond appropriately..

1. the fact that he asked for the picture BEFORE he knew you were in the picture speaks volumes..you guys weren't together, so really you can't be mad at her about that.

2.it wasn't a nude picture & yes obviously with him being in jail..he most likely would be beating off to the picture no doubt..but again..this is before you came along.
3. she never sent the picture and then informed him that because you guys are together she didn't..that shows quite a bit of respect i would think..there are many women these days that would have sent the picture anyways behind your back under the guise of " he can't tell me what to do, he's my friend" she didn't do that. she was upfront and told you everything.

4. i wouldn't worry about comparisons..she's with you, you are with her and he is in jail..hmmm..enough said there.

5.now in all honesty "just friends" means something different to us guys as opposed to women(we know that) & truthfully i have slept with some of my ex's after we broke up..but first he is in jail so not likely unless somehow he can evade security guards, bars and cameras..not likely.second she is being upfront and telling you everything,which if there was something to worry about would be if you found out from a secondary source.

6. I admit i wouldn't be happy about my gf seeing an ex in jail..i'm not gonna lie..i wouldn't, but i would tell her that i wouldn't be comfortable with it and put the ball in her lap..never would i say "you can't go" or "i'm not going to put up with that kind of disrespect" and i admit i would ask her why it is important for her to go visit an ex, when it is us that should be an item..i will say that. i have an ex that is in jail right now and has been there a few months..my gf at the time forbade me to go see her and i complied..it's a sticky one there, it is. so truthfully i probably would ask her not to..i wouldn't command it or make it a condition of us being together.

that area is a slippery slope..do you talk to any of your ex's?..for some reason that always seems to cause problems..it's hard to say because each relationship has a different dynamic.

i do detect a tone of insecurity and jealousy in your post my friend.i think it is much ado about nothing to be honest .idk 1 visit would hurt, multiple visits...ehh,idk about that..but that's JMHO.