OMG. That one was on the first few pixelart pieces I studied back in 2006/2007 :P ...
Not that I had learn much from that specific piece, but I surely remember the mad squirrel ...
We had a similar issue with someone else using most of our Pathway release trailer in their trailer and stitched their own anime characters on top it... (Plus some more stolen work) We tried to get it removed but it appeared again and again for a while... :/
If you paid for any sort of company Trademark / Copyright you can tell them that in the complaint and get that stuff shut down much faster. Sucks, but my QuestLord TM has shut down things within a day. I don't know about Steam though.