The Diathesis-Stress Model

In order to understand the media's influence on body image and more specifically on the development of eating disorders, you need to have an understanding of some basic psychology.
Like most disorders Anorexia can be explained by two factors, genetic predisposition and stressful life situations.
People who develop anorexia often share similar psych profiles, and personality traits the most important of which is a tendency towards secretiveness.
These sets of genetic predispositions which include personality traits combined with environmental factors resulting in a disorder is called the Diathesis-Stress Model. This means that there isn't a single factor that results in an eating disorder nor can either factor in isolation result in an eating disorder. This explains why not all people with genetic tendencies toward these disorders develop them and why not all people who have stressful life situations develop them.
The media and more specifically the fashion industry would fall under the stress aspect of the development of an eating disorder.
When women who are not healthy continue to be placed in a position where they can be viewed and idolized by young women it creates an unhealthy belief about what a woman's body actually looks like. This reinforces peoples belief that they can become a fantasy that does not exist.