How do I change my gamer picture on xbox 360 to something like The Little Mermaid?

I have been trying to download pictures online and on the xbox marketplace but I can't figure it out. I'm tired of that dumb panda.How do I change my gamer picture on xbox 360 to something like The Little Mermaid?I don't know how but why the little mermaid? Hahaha that's not gonna get you very much respect from other dudes.