Wow, this is a BIG question and literally hundreds of books have been written on the impact of media on the development self-esteem, self-worth, body image for kids/teens and adults. Because we are visual creatures and the new media (which of course includes the internet and Social Media) is all about visuals, we are bombarded and bathed in images 24/7. Each image we see of a human causes us to judge that image as "Good" or "Bad" and of course, we compare how other people look with our self-image. Most of the time, tweens and teens are hyper-critical of themselves and so you can see with a steady and abundant diet of PhotoShopped lovelies that a young person's self-esteem is fighting an uphill battle.

We can't change the media, but we can help tweens and teens become more savvy consumers of these media messages. I've got several helpful books and sites to recommend. Ready? Here goes:

About Face http://about-face.org/ Their mission is to equip women and girls with the tools to understand and resist harmful media messages that affect self-esteem and body image.

"So Sexy So Soon" by Diane E. Levin. A book about the way media and the marketing arms of huge corporations influence children in unhealthy ways. This link includes my podcast interview with Dr. Levin.
http://blog.anniefox.com/2010/01/17/podcast-so-sexy-so-soon/

Shaping Youth: Amy Jussel's excellent site/blog about the impact of media on young people. http://www.shapingyouth.org/

Education is a powerful transformer of attitude and behavior.

I would say the media has a large affect on teen body image. When teens watch shows with characters they resonate with, they can consciously and unconsciously associate these body characteristics with what an ideal person looks like.

More info can be found below: from Education.com and The Neumors Foundation.