I've jailbroken mine to have custom covers. Instead of famous dead American authors I've never read, I have famous dead physicists whom I've actually read and whom I admire.
I'd love to do this using "pulp" science fiction book covers. I assumed that there was already a tutorial for this, but my Google-fu is weak. Did you use a guide or just DIY? What size/resolution image does the Kindle expect? Is there a filename format or a directory structure to use?
Image format is grayscale 800x600 (bigger for Kindle DX) png or jpg. For best quality I recommend fiddling a bit with the color to grayscale work in Gimp or Photoshop, especially if the image has large areas with soft gradients. IIRC, running a selective Gaussian blur after grayscaling, and/or working the curves tool a bit before grayscaling, will improve problematic images quite a bit. Early models had only 4 grayscales, now they have 16. "Posterize" to 4 or 16 colors after grayscale in Gimp will give you a good idea of how the end result looks.
Some people add their contact details to the screensaver images, so if you lose your kindle your contact details are the first thing someone who finds it will see.
Maybe a better hack would be to disable screensavers since they serve no purpose on an e-paper display and actually cost battery to switch from text -> screensaver -> text.
The better behavior would be to not flip the screen at all and keep it static, using no battery.
