Backgrounds after the jump...
First off we have this promotional image from Grand Theft Auto: Vice City Stories. What PSP wouldn't look better with this on it?
This next background also comes from a promotional image, but this time it's for Metal Gear Solid: Portable Ops.
And lastly, because Andrew and I are both addicted to it, I decided to put in a background for Death Note, which is a manga/anime/movie/phenomenon. If you haven't seen anything about Death Note yet, maybe this background will get you interested.
To put these as your PSP background, you can either save them on your computer and transfer them to your PSP, or if you're browsing this site on PSP, you can save them directly and when you're viewing the image on the PSP just hit the triangle button to bring up the image options and select "Set as Wallpaper".
Feel free to e-mail in suggestions for what you'd like to see in future updates and enjoy!
[Update 1: Fixed image size issue]