I tried messing around with this and here's what I found:
- A "Kindle Preview" generated MOBI file, sourced from an imported EPUB, shows the thumbnail correctly after sideloading on my Kindle, but the sleep screen doesn't show the cover.
- An AZW3 file converted online from either the MOBI or the EPUB shows the cover on the sleep screen, but doesn't show the cover in the library.
Unpacking the MOBI and AZW3 files, I noticed that
mobi8/OEBPS/content.opf contains a reference in the metadata, to a "Cover ThumbNail Image" which is not present in the
mobi8/OEBPS/Images folder (but IS present in the
mobi7/OEBPS/Images folder).
I also noted that all the online services that I tried that convert to AZW3 have a <dc:contributor>calibre (5.37.0) [https://calibre-ebook.com]</dc:contributor>, so it looks like they use Calibre to perform the actual conversion.
I tried converting using Calibre 5.43.0, and a MOBI file had no sleep screen cover, and an AZW3 had no library thumbnail.
I
didn't use Calibre to transfer to the Kindle, just copied the file over to the Kindle Documents folder.
I can unpack an AZW3, but don't know how to repack it, so... could anybody try adding the thumbnail image to the
mobi8/OEBPS/Images folder of an AZW3 file to see if the thumbnail shows in the library?