It depends on the game, in general I use the cover art for the game then tack on some number. There's sometimes when I'll make something that looks kind of like a cover art but with some tweaks and sometimes I'll make ones out of something I said.
Some examples being my Fallout New Vegas thumbnails, my Fallout 3 thumbnails and the Weed mission from Far Cry 3.
I'd just put them here but I deleted them a few days ago.