[tshering]

For test purposes, I upload here a package that provides random selection of the poweroff screen image.
After installing the package, select "scripts > poweroff.sh" in order to test how it works. For this test package I borrowed the images from the kobolauncher sleep folder. They are now in .kobo/kbmenu/poweroffimages. You can replace them by your own jpg or png files. The file names must not contain spaces.
If you are content with the results of your testing and wish to use it for real, copy the file poweroff.sh from kobo.\kbmenu\scripts to kobo.\kbmenu\onstart (replacing thereby the original poweroff.sh file).

This is only for KSM version 06.

Edit: It is preferable if the ratio of the images corresponds roughly to the screen ratio, the actual dimensions are not so important. Real large images might possibly cause problems. I tried 2136x2848 / 3 MB images without problem.