[Lynne]

Could do a plugin:
hook location - postbit_display_complete

PHP Code:

if (is_member_of($this->post,x)) {
$this->post['avatarurl'] = "http://yoursite.com/path/to/avatar.jpg"; 
$this->post['avwidth'] = 'width="100"'; 
$this->post['avheight'] = 'height="100"'; 
} 


replace x with the usergroupid and enter the correct path to the url and the correct width and height. That should work for the postbit (I think, I haven't tried it). Just do similar for wherever else you use avatars.