I'm going to do some searching for images and see what I can come up with. In the meantime-- do you think that using implements of increasing degrees of sucky-ness would work? So for the top, they get to spank with a more severe implement the more they post, and for the bottoms, we get spanked with a more severe implement. Just a photo of the implement?
But that's kind of dull, maybe.
Maybe (like the default "top" image you have right now, which is waaay cute) we show images of the spanking, with the different implements? But then the implement would be too small to see...
Okay, now I am thinking out loud (well, on keyboard) and kind of getting nowhere, so maybe I should shut up and actually think about this for a bit!