Lulz you can do that without a plugin. You have to assign a user rank image to a usergroup. You log into your admincp and on the left side expand user ranks like such:
Click on "Add New User Rank" and you should get a page like this:
Now you add whatever you want I'm going to set the administrator user rank like such (you can have it the way you want but this is how I do it):
(The path depends on where you put the image!)
Alright after you save it or add you will see your user rank in the manage user ranks page like such:
Also, YOU MUST COMPLETE "Update Counters" which is located at:
your-url.com/path-to-forum-or-admin/misc.php?do=chooser
Once you do some of the updates it will work for the user groups done. I suggest you add all user rank images before updating everything.