We bought Kindles a while back; and while the default set of wallpapers probably appeals to someone, personally I'm not into the "slightly creepy images of dead authors" aesthetic. So, I created my own set using photos I've taken of street art in Brisbane, Sydney and Melbourne.
These are free to download and use if you share my "australian stickers and graffiti on your kindle" aesthetic - I'll admit this might be a niche market :) They're available separately on Flickr or you can download the whole lot (17mb zip).
To use these you need to jailbreak your Kindle and then install the screensaver hack. The trickiest bit is working out precisely which version Kindle you have so you know which files to use (go by the serial number). Note that once you copy new screen saver images onto your Kindle, you need to restart the Kindle before they'll start showing up (home, menu - settings, menu - restart).