Ok looks standard userinfopane. So try adding this code just above the final < /div > like this (presuming the member id of site administrator is 1) Change to the correct http address of this special avatar...
<if test="$author['member_id']==1"><ul class='custom_fields'><li><img src="address to avatar"></li></ul></if>
if you want to do it by groups like root admin (4) and admins (6) use this
<if test="in_array($author['member_group_id'],array(4,6))"><ul class='custom_fields'><li><img src="address to avatar"></li></ul></if>