[TECK]

Pretty easy. Create a link with the usergroup in your postbit template...
Example:

<img alt="<if condition="$post['usertitle']">$post[usertitle]</if>" border="0" src="$stylevar[imgdir_misc]/usergroup$post[usergroupid].gif" />

Then, simply upload into your /misc dir your usergroup images.
For Admin, it will be usergroup6.gif, etc. you get the idea.

EDIT: In case you want to display it only for Admins and SuperMods:

<if condition="$post['usergroupid'] == 5 OR $post['usergroupid'] == 6">
	<img alt="<if condition="$post['usertitle']">$post[usertitle]</if>" border="0" src="$stylevar[imgdir_misc]/usergroup$post[usergroupid].gif" />
</if>