I think I got it now... and It's not of Sandy's posterior
It IS NOT a picture of anything lewd. I can definitely see a bit of the female form (torso and breasts) on the right of the art.
I can imagine the part on the left of the picture is of one of the curtains, the window behind it, and of course sandy topless.
Swampy messed with a ton of photoshop filters and such and sort of morphed parts of it into looking like an outline of something abstract.
Swampy might be a bit of a slacker and kinda out there in the last few strips. But I don't think he's enough of a perv to take a picture of his girlfriend's butt, then get it printed out at Kinko's and hung on her wall. Swampy has some class
Dare I say that this kinda says something only hinted at in a wallpaper that Doc did. That Swampy is a very talented artist. The wallpaper of Swampy carving a beautiful ice sculpture with a chainsaw of Sandy with significant embellishments done to it.