[Cryo]

1. Admin CP -> Usergroups -> (Select Usergroup) -> Username HTML Markup

Simply place...

<img src="http://url.to/image.png" align="absmiddle">

... in the first field. This will show whatever icon you would like almost everywhere their username appears on the forum.

2. If you only want to do this in postbits you could do something like...

<if condition="$post[usergroupid] == $bbuserinfo[usergroupid]">
<span style="color: #FF000">$post[musername]</span>
<else />
$post[musername]
</if>

The problem with this is if the marked up username is a different color I believe it will overwrite the span color. You could alternatively use the $post[username] variable, but then you lose the icon... unless you manually edit it in.

Hope this helps!