If you don't want to use koreader, though it's got it's benefits, there is this option
https://www.mobileread.com/forums/sho...d.php?t=255238
Works with the current firmware. It does need a little tweaking to be properly used with any device which isn't the H2O, but once that's done it should be smooth sailing. It'll display a random image from a folder, or if you only put one image it'll use that one. It does this for both the sleep and power off modes, each having their own folder.