@NiLuJe
thank you for your detailed reply. now i understand how the cover view works, i think

. let's see if i got it right:
1. if there's a cover in a mobi file, it's added when the file is created.
2. Calibre can add a cover to a mobi file, but this will not be bundled into the file and will not overwrite the original cover. Kindle will use it as the cover, but this Calibre created cover may not be seen in other e-reader such as mobipocket reader.
3. JBPatch uses the thumbnails for it's cover view.
a few questions, which folder are the thumbnails stored? are they in a viewable format?
when i tried out the "last thing" mode, i encountered a small problem. it seemed that the "last thing" screensaver was very "stubborn". i changed to "cover" mode and rebooted my KT, but the "last thing" screensaver was still there. i even deleted the png file in the screensavers folder, but still couldn't get rid of it. it was only after switching to different covers a few times that i was finally able to activate the "cover" mode.