I download epubs, they already come with covers.
In Calibre, if I donīt like a cover, I change it and use a jpg the size of my kindle PW (1448 Ũ 1072 pixels) or leave the original cover they already come with (800x600).
After, I run the polish books function, with the update metadata and update cover options on.
I convert them to azw3, and transfer them to my Kindle PW4.
Disconnect kindle, then reconnect to PC with kindle open (so the kindle doesn't change covers), and disconnect again.
I've doing that to all my books since before anyway, and now they show fine with the cover on the kindle. 800*600 looks ok on the kindle (with black bar at the sides), but 1448*1072 looks better ofc.
I donīt have to mess with asin, isbn or anything, since my epubs don't come with that.
*just one time I had a problem with one ebook not showing the cover, so I deleted it, changed the cover with calibre, did the steps above, reseted the kindle, and it worked.
That's my two cents.
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