While I initially liked the ability to set custom backgrounds on my iPhone 4, I've come to find that I'm starting to miss the simple black background I had on my iPhone 3G. How can I set my background back to black?
Create a black image in your favorite image editor (even Paint will do if you're on Windows), save it as a jpeg, and e-mail or sync it to the phone. Then set that image as the background.
Since you're making it all black, I doubt the resolution is going to matter much, but you can always use 960 x 640 to use the iPhone 4's standard resolution.
Edit to add a black image that works: