I can make a tutorial for this, but it may not be for the same way you are thinking.
Currently, we don't have the feature to add your own rank images, but we do have
a large list of rank images and stars you can use, located here (http://www.smfforfree.com/starpreview.htm).
Does this (http://www.support.smfnew.com/index.php?topic=9470.0) answer your question?