I modified it a little to work with custom avatars stored in the file system. Might not be the best way but it works, hopefully this helps out.
Code:
$member_count = 3;
ob_start();
require_once('./includes/functions_user.php');
require_once('./includes/functions_bigthree.php');
// Get Random Members
$newusers_get = vB::$db->query_read("
SELECT ".TABLE_PREFIX."user.userid AS userid, ".TABLE_PREFIX."user.username AS username, ".TABLE_PREFIX."user.avatarrevision AS avatarrevision, ".TABLE_PREFIX."customavatar.dateline AS dateline FROM ".TABLE_PREFIX."customavatar
LEFT JOIN ".TABLE_PREFIX."user
ON ".TABLE_PREFIX."customavatar.userid=".TABLE_PREFIX."user.userid
WHERE ".TABLE_PREFIX."customavatar.visible = 1
ORDER BY RAND()
LIMIT $member_count");
$output_bits = '<p align="center"';
while($newuser = vB::$db->fetch_array($newusers_get))
{
$output_bits .= '<a href="member.php?u='.$newuser[userid].'"><img src="/customavatars/avatar'.$newuser[userid].'_'.$newuser[avatarrevision].'.gif" alt="'.$newuser[username].'"/><br />'.$newuser[username].'</a><br />';
}
$output_bits .= '</p>';
$output = $output_bits;
ob_end_clean();