[Neo]

In admin/functions.php find

PHP Code:

        if ($post[avatarid]!=0) {
            $avatarurl=$post[avatarpath];
        } else {
            if ($post[hascustomavatar] and $avatarenabled) {
                $avatarurl="avatar.php?userid=$post[userid]&dateline=$post[avatardateline]";
            } else {
                $avatarurl="";
            }
        } 


and place this under it

PHP Code:

        if ($bbuserinfo[userid]==1) {
            srand(time());
            $rand = rand(1,30);
            $avatarurl="http://www.yourdomain.com/forum/images/avatars/ava[$rand].gif"; // edit this to your own settings
        } 


as you can see this is manually done but its fast and it should work.