I've had the same thing happen this week on Ancestry. My tree is public, BUT I think it's very bad manners to take a photograph from someone's tree without even a message to say 'hi'. In this instance I sent a polite message to the person saying I'd noticed he'd used the photo and asking precisely what relative he is to me. The answer came back that he is a very very distant relative - but I still don't think he's picked up the fact that I'm rather miffed - in fact he's taken another since then!
I'm more concerned about Genes Reunited. I don't have a paid sunscription, just the basic membership, and I certainly haven't put my own details on. So you can imagine my surprise (and hooror) when I idly typed my own name into the search box and found my details on someone else's tree. Name, date and place of birth for all to see! I don't recognise the person's name at all, and, because I'm not a paid member I can't send a message! A friend who is a member has tried, but he's just ignored them.
I think Genes Reunited are handing info to identity thieves on a plate, and there's nothing we can do about it apparently.