Nice to see an older sister looking out for her brother :) As for the question, I have mixed opinions on it. Really it depends on how often she sees the guys and what sort of relationship she has with them, which I certainly don't know and you may not.
Yeah, I don't agree with that. It makes it look like she's single or going out with one of these other guys.
My boyfriend used to talk about hating his ex girlfriend - Turned out there was a few reasons for this including that she'd once posted a picture on fb of her and a guy friend and made it look like THEY were going out. She'd talk to her ex when she was with him too and would save his pictures so he thought she was a player.
I wouldn't be comfortable with my boyfriend posting a picture/s with other girls (unless it was like his sister or something), and wouldn't post pics with other guys. There's just something not right about that. I'd warn your brother to be careful on this one.
It seems to have a lot of un-loyalty written on it to me
On the potential list of things that I care about, this ranks very low. Taking pictures with your friends is not abnormal. Having platonic male friends is also not abnormal. It doesn't mean she's cheating on your brother.
Men get so jealous, there is nothing wrong with taking a picture!
Ask her boyfriend if it's ok with him, like casually. If he isn't, he'll he confronts his girlfriend, if he is satisfied his response then it's for the guy and the girl's decision. If he trusts her, this shouldn't be a problem.