That image was from the infamous 2009 April Fool's Gag. (If you tinker a bit, you can find a full-body version, also topless. )
The gag pics in there were roughly converted copies of old sketches, such as the one I used for one of the early T-shirt preorder sales. They look good squashed down as they are, but the full-size versions are rough and ugly.
With very few exceptions, you guys have seen damn near every lick of TWB art I've ever drawn.
The main exception is, of course, the book sketches. I've done somewhere around 300-400 of these over the years, and while I've shown quite a few at times as sketch filler strips and the like, I've by no means shown them all. (The faint background image on TWB is all book sketches.)
But none of them are porn, and few are even as 'suggestive' as what's shown at times in the regular strip. (Which has, admittedly, at least once shown both Sandy and Pirta completely nude. )
Most of the rest is just junk- an alternate Book 2 front cover that never got finished, that "Gangsta" shot of Rainy that readers wanted to see as a T-shirt or something, but I could never get to look right, some holiday graphics that didn't turn out how I wanted, that kind of thing.
I've been meaning to put more of it up under the "sketchbook" page, but as always, time is against me.