For those who have been following this thread but are finding it a bit manual / complicated, I would encourage you to check out this plugin that can automatically convert files in your library to KFX / PDOC so that the covers can be displayed. You also need to install the KFX Output plugin (just click on Preferences -> Get plugins to enhance calibre) and the amazon kindle previewer 3 software (
https://kdp.amazon.com/en_US/help/topic/G202131170)
Plugin is here:
https://github.com/scotjam/calibre_kindle_cover_fixer
(Install using Preferences -> Advanced -> Plugins -> Load plugin from file and choose the zipfile release from github above)