1) Copy the image you like to use (in our example pb_admin.gif) into the images/misc directory of the styles
2) Enter the AdminCP, Styles, Style Manager
3) Edit Templates
4) Postbit Templates, postbit_legacy
5) Search for
<if condition="$show['avatar']">
<div class="smallfont">
<br /><a href="member.php?$session[sessionurl]u=$post[userid]"><img src="$post[avatarurl]" alt="<phrase 1="$post[username]">$vbphrase[xs_avatar]</phrase>" border="0" /></a>
</div>
</if>
below add
<if condition="$bbuserinfo[usergroupid] == 6">
<div>
<img src="$stylevar[imgdir_misc]/pb_admin.gif" alt="Administrator">
</div>
</if>
The condition displays the image if the user is within the usegroup 6 (default the administrator group). You could see all usergroup id's in the AdminCP under Usergroups, Usergroup Manager.