I noticed a bruise on her palm in her profile picture. How do I ask her about it?

[Renrenrenren]

[quaranta]

I don't think you need to ask her about that, it's just a bruise, it's not like having a bruise means she got an actual incident, or violence. People just manage stuff, somtimes fall down, sometimes get some hits here and there. If I had to justify all my bruises...
So unless she posted the picture of the bruise on purpose to show the bruise as the subject of the picture, there is no need to ask about that and she probably wasn't trying to get that kind of questions. Most likely people who are close to her already asked her about it (irl or not) and made sure it was nothing serious.
Also I don't think bruises are a good conversation opener if you were looking for an excuse to start a conversation, however you can try asking anyway but that might give it away instantly that is just an excuse. Because if you don't have a bond with her it's unlikely you genuinely care "that much", imagine if it was a guy with the same level of closeness you have with her, if you asked him about his bruises he would probably go in "wtf".

[Guy13]

Do not ask or she will never maintain relationship with you. No one likes to be scrutinized to this degree. Lighten up and focus on what she is broadcasting to you to focus upon.

[Renrenrenren]

But I’m worried about her, are you sure I shouldn’t be asking her about it?

[Guy13]

Get a grip on your emotions before you start something that gets out of hand. Pay attention as temperatures warm. If you see grab by arm bruises then slowly work yourself into her Business and get the retarded story shot back at you.

[FunkyMonkee]

Say, bitch, what's that bruise on your palm from?

[Dinosaursandanime65]

Say “did a pole cause this?” as you point towards it.

[In_Trance]

WTF is up with your hand?

[SnowedIn]

Don't.