$userinfo[avatarid] should hold the user avatar id.
$avatar[avatarpath] should hold image path.
you'll need to add plugin that will query for the info, and then assign it to some var, and place the var into the template.
I would place the code into the "forumbit_display" hook.
Also, check this link:
http://www.vbulletin.com/forum/showthread.php?t=197582
might help you with your coding.