here is a comic i found on deviant art. I just happened to be looking up Powergirl and saw what looked like some familiar art that was similiar to a piece of art i saw some time ago that involved powergirl getting her but tickled. So i checked out the artists site, and found the full comic, which had some additional ass tickling scenes. Worth a look.
http://bikerbloke.deviantart.com/gallery/?offset=24
http://bikerbloke.deviantart.com/gallery/?offset=24