As per my post in the Jailbreaking Kindle 4 thread I am using jpg greyscale images. The file names are not the same as the original names e.g. bob_600_800.jpg etc and these work fine when stored in the /mnt/base-mmc/opt/amazon/screen_saver/600x800 directory.
As for their names, the screensavers are shown in alphabetical order and I'll bet the marketing guys made them change the order and images multiple times, so instead of renaming all the files they simply renamed the filenames.
I'm pretty sure 08N.png was this:
... but that didn't last long