How do you change you background on your youtube channel?

like this http://www.youtube.com/user/Thewantedmusic i know they are famous but there must be someway of doing it? or like this http://www.youtube.com/user/AstridTR ? please help, thanks. my youtube channel is http://www.youtube.com/user/megamindme?feature=mhee and its boring so i would like to personalise it with cool pics of me and friends! thanks!