You have to install the PHP in templates plugin. If you don't have it, download it here: http://nulledbb.com/Thread-PHP-in-Templates-2-0
Navigate to your postbit templates: AdminCP > Templates & Style > Templates > Postbit Templates > postbit or postbit_classic
If you want to add the second groupimage to one user add this below {$post['groupimage']}:
Code:
<if $post['uid'] == 1 then>[img]path/to/groupimage.png[/img]</if>
If you want to add the second group image to more users, add this below {$post['groupimage']}:
Code:
<if in_array($post['uid'], array(1,2,3,4)) then>[img]path/to/groupimage.png[/img]</if>