I would personally say:
Dear Judgmental Prick,
If those guys aren’t sexist judgmental jerks they would respect me for who I am regardless of what I wear.
Love,
A girl who has her boobs hanging out in some pictures.
That’s a little confrontational of course, but that’s the type of person I am.
Honestly though, most of the time it’s not even worth it to say anything. I just unfriend people who post shit like that.