Edit viewtopic.php with a simple if statement, along the lines of:
if ($cur_post['g_id'] == '<group's id>)
$username = '<a href="profile.php?id='.$cur_post['poster_id'].'">'.pun_htmlspecialchars($cur_post['username']).'</a><img src="img's location" />';
else
$username = '<a href="profile.php?id='.$cur_post['poster_id'].'">'.pun_htmlspecialchars($cur_post['username']).'</a>';
Ofcourse you could go a bit more complex with it, but it's a quick fix.
[img]http://i192.photobucket.com/albums/z314/avantgarde--/helplol.png[/img]