If you have a firmware 5 kindle (Touch, Paperwhite, Voyage, Oasis), see the screensavers hack thread on Mobileread:
> Dou you prefer to just show the last thing that was on screen, with an overlay indicating when the device's alseep? Just drop a blank file named last in the linkss folder. Restart your Kindle ([HOME] -> [MENU] > Settings -> [MENU] > Restart; or simply use the Screen Savers > Restart framework now button in KUAL), and you're done