Different perspective here...

At this point in time, you really could just be reading way more into this than there really is. She could've honestly agreed to send the pic to him strictly as a friend.

Something beautiful to look at while he's locked up. Seems plausable to me, since it wasn't a request for a nude, which would definitly be wayyy more than friendly.

If a tank top and a shorts is a no no pic, well then idk, I guess I have some shorts and tops to throw away as according to your feelings regarding a pic suggests these are not "respectable" clothing for me to be wearing out in public.

The fact that she changed her mind once she had a bf, because he would take issue with it, counts in her favour too.

So in essence the pic shouldn't even be an issue because there is NO pic. She put up stricter boundaries around their friendship, boundaries that are bf friendly once you entered the scene and you should take that into consideration when you're stressing yourself and her out about this topic.

You dictating whether or not she can visit him however, does not count in your favour.
Not at all.

It makes you look extremely insecure, jealous and controlling. Its not like they can actually DO anything besides talk during the visit, so no reason to distrust her visiting him in prison. Her visiting him is NOT a sign of disrespect to you.
I have NO clue where you're getting that idea from, but you dictating who she can and cannot see, who she may and may not be friends with?
Now THAT IS disrespectful.

Idk. Maybe I have it wrong, but if she's given you no actual reason to distrust her, except be friends with an ex (like so many of us are) then you will only sew the seeds of the distruction of your relationship by continuing on this path.

Exes can be friends, sometimes though, due to the familiarity between them, the boundaries of these friendships may not be as rigid as when these friends are in relationships.

That's not to say these exes are sleeping with eachother, but just that maybe there's jokes of their shared past, trips down memory lane, sharing of pics or whatever a bf or gf may have a problem with in the context of a relationship, but doesn't actually overstep the bounds of being platonic.

The important thing is to reinforce these strict boundaries when there is a SO in the picture, because it can definitly be inappopriate and disrespectful.

Something your gf has clearly done within the context of your relationship. So I believe she deserves the benefit of the doubt.