How can I represent Chapter 18 in a different "media" format. Like a picture, movie poster, comic strip, or newspaper.
The representation has to somewhat explain the chapter and its relevance to the novel.
In chapter 18 Holden dislikes both the Rockettes show and the war movie he goes to see. What is important here is why he dislikes these shows, and why he is such a cynic in general. Holden has difficulty connecting to others because of his own insecurities. His cynical attitude is a form of self-protection.
You can present these ideas in several ways, just choose a "media" format you're comfortable with. For example, a comic strip can show a character's inner thoughts. You may want to provide more of a metaphorical representation with a picture. You could also have Holden interviewed in a newspaper article. Be creative and play to your own strengths.
Review the chapter and ask what you feel is most significant about it. A movie poster could depict the show Holden sees at Radio City. He, of course, hates it and sees it as cheap and gaudy. Since the movie he sees is about war, he decides he's against war, but for what reason? He opposes it only because he would have to fight with men like Ackley and Stradlater. What does this say about Holden's values?
This scene could also be shown as a comic strip. In this format, you could reflect Holden's thoughts as a character and show how he views the movie, war, etc.