First thing is to block those sites from her iPod.
Take her to the gym to talk to a fitness coach. Sometimes, hearing about it from a "professional" means more than coming from mom.
Also... I wish I knew what the website was, but I'm sure you can find it on Google. Basically, it shows the differences between real-life and published photographs... On the same models. So you see the model looking just like an average, everyday person (and some of them have "worse" bodies than you see in the slam section of those celeb magazines.) then it shows how much the makeup changes them, then how much photoshop is done to achieve the desired look. It was quite they eye-opener.
Also look up images of girls who have these eating disorders... Point out that their hair and skin SHOW how unhealthy they are, and how unattractive it really is to allow yourself to get into that state.
You can also look up health information about the effects of these disorders... Mortality rates, psychological effects, etc.
Many girls see these disorders and think that they are not that serious... Just a way to lose weight. They don't understand that once they get started down this pay, it is very difficult to pull out of it. Definitely try to nip this in the bud.
You could compromise with her a bit, by allowing her to have some input in meal preparation (don't go low fat, but maybe healthier options for certain things...) maybe get her a gym membership, where she can improve her physique in a HEALTHY way. Of course, these things may be tricky to do, because teen minds... They are weird. She may think that doing these things mean that you secretly agree that she is fat and needs to do something... But then you worry that by NOT compromising in some way she will think you are totally not listening to her, and just moming out. You will have to have a good talk with her and decide what course of action to take.