Find this in your postbit (or postbit_legacy):
Code:
<a class="bigusername" href="member.php?$session[sessionurl]u=$post[userid]">$post[musername]</a>
Change to:
Code:
<a class="bigusername" href="member.php?$session[sessionurl]u=$post[userid]">$post[musername]</a><if condition="$post[userid] == 'xxx'"><img src="$stylevar[imgdir_button]/yyy.gif" border="0" /></if>