I'm a big fan of "piggyback art", taking an existing piece of art and adding something to it (like when I added Cthulhu emerging from a cave to the background of a boring $2 garage sale landscape painting then hung it in my living room and saw how long it took people to notice).
Photoshop master Matthew Buchholz sells some awesome piggyback art in his Etsy shop, Alternate Histories. He takes public domain maps and photographs then adds monsters, robots, aliens, zombies, and other sci-fi tropes to them.
We've placed several of our favorites after the jump (click pictures for monstrous size). Man, this is making me so nostalgic for the Annual Zombie Killing Games of Los Angeles.
[Hat tip and a Domo-Kun daguerreotype to Nerdcore.]