It seems that some folks believe that a bit of the art borders on "parody" or "rip off".
My take is "inside joke," "homage" or "Easter egg."
You see, some of us old timey gamers will see this stuff and we'll recognize it's original origin. It puts a smile on my face. Newbies will just see "art."
I was just leafing through the PDF again and this little number popped out at me. I've looked at it a half dozen times already, but it finally clicked:
It is an homage to "The Keep on the Borderlands." Here's the original: