As Wibbs said, the avatar function on the forum is turned off and is unlikely to ever be turned on as neither Niall nor any of the mods like the idea of you lot being let loose with them.
The odd posters who have images under their names have them in html tag lines which only the mods can set. They were generally done either in payment for favours (Twisted Woody, Fred, Moz and others got Keano ones for answerinng Niall's request for Keano articles, Baz for donating to the forum etc) or in the case of Weastie, Jason & Cnut for demonstrating they lack a life by reaching 50,000 posts on here.