To choose an avatar, you have to go there http://www.killermovies.com/forums/...tion=editavatar (pick one from the list or upload one from your computer)
To make a signature, go there http://www.killermovies.com/forums/...ion=editprofile (in the signature section of the page there is a link to upload a picture)
For other question, please go to the FAQ section : http://www.killermovies.com/forums/misc.php?s=&action=faq