I've been using Google's image search as my wallpaper finder here lately.  Here is how I do it.

Step #1 - Search Google for something, for this example - I picked "mario".

Step #2 - Click on the "Images" link and then, "Show options...".

Step #3 - Click "Exactly..." in the left side menu, and then put in your screen resolution.  Hit the search button. 

You should get all kinds of fun results in return. Oh, and just in case anybody's wondering - I decided to go with this wallpaper background.

Hope you enjoy!