For the Amazon-compatible "azw3" files that I'm seeing, I'm curious why the book cover thumbnail images are a separate download from the ebook file itself? Unless I'm missing a trick, it seems like you have to use Calibre (or some other application) to re-build the "azw3" file with the cover thumbnail properly embedded. Why not just ship the ebook files like that to begin with?
>>>Thanks to a long-standing bug in the Kindle software, side-loaded ebooks don’t display cover images automatically. "