how to assign different default images to different roles
If you want to display an image for a particular user role , say "Role1" , then in page.tpl  

 &lt;?php global $user;
  if($user-&gt;roles[2] == 'Role1')
 {
 print $logo;
 }?&gt;


Just print the $user to get in what values your role is in the role array.

If you print $user, your role may be in this,

 [roles](
 [1] = role1
 [2] = role2
 [3] = role3
       )


So choose your role and check the condition and accordingly print the $logo however before that get the image in $logo using the image source tag.