when you go to www.g20.net/forum there should be a welcome screen with an icon on the left - click that to enter the url for your avatar. You have to have the image you want to use hosted somewhere tho.
Also you can set it up under "user cp" up under the banner. Go to "edit options" then scroll all the way down to change your avatar. Like mentioned earlier however, you need to have the image hosted somewhere else online, and it must be 100 x100 pixels or something like that. Also, the image can't be over 20000 bytes in size.