I'm not a professional at what constitutes as YA, but I think it depends on how your describing her. Teenagers are familiar with sex (I'm sure we all know that by now). Many of them are having it, if not, they are usually pretty curious or discovering their sexuality or something along those lines, so sex isn't something you have to worry too much about with teenagers (of course, this is coming from a twenty-something who does not have teenage kids herself, so parents might object). I think if you're going for a sexual sort of thing, it's probably best to keep in mind "less means more". Not that I don't think teens can't handle a naked body. They're well passed the age of cooties, but as far as parents go, some of them are a little stricter than others, and because of that, I'm not sure how well that will pass with an agent or publisher. But, again, I'm not certain about these things.
If it's for an artsy sort of thing, I think its fine. At that point, I think of it more like those sculptures you find in museums, and no ones hiding their kids from there, so I think it would be fine.