How does the media effect body image of young girls.?

Negative expectation. Media uses idealized, not necessarily realistic, healthy or safe, images in order to sell their goods & services. In the case of women, they're all thin w/large breasts, snow-white perfect teeth, blond hair, blue eyes & bright red lips. Thus, every little girl wants to grow up to be this person. We hold the power in that we decide when to open the pocketbook. Don't buy what they're selling.
1-D Standards Beauty. "the media is a multi-billion dollar industry that often portrays young girls in sexualized, objectified & one-dimensional ways that do not account for the range of body types & shapes. Girls often evaluate themselves against these external standards of beauty that become assoc w/ glamor & acceptance.” source dr julie ancis, apa fellow. Author "the complete women's psychotherapy treatment planner".
Perfection. The media shows picture perfect people with great bodies and it gives the impression that you can have their life if you have their image. Its used to sell products as well. Unfortunately, girls strive to have that body image of what they see and can starve themselves, or vomit to achieve it. It can lead to eating disorders and distorted body images with altered self esteem.