Hmmm.... how d'you feel about used back/foregrounds?
I was making a Yoshi fangame about a year ago... I ended up making the movement engine and a little test area, then abandoning it.
So, anyway, I have tile graphics for a tropical area, both for the foreground and the background.
...Of course, they might also be in such a format that you wouldn't be able to use them... you see, each one is in a tileset, a single image of every possible 16 x 16 tile that could appear in the game. That, and the background was intended for parallax.
So, um, the bottom line is, I'm no longer using them. Would you have any use for 'em/be at all interested?
Go Moon!