Well, hello again!
Sorry, you're wrong. And no, you can't.
Amazon KDP requires the marketing image to be uploaded separately. And regardless wether your ebook contains an embedded cover image or not, Amazon will create a new one from the required marketing image (which, again, was uploaded separately).
The Kindle and Nook handle cover images very differently. But one thing they do have in common: the marketing cover image must be uploaded separately. Neither of them will extract it from your ebook file.
Kind regards, David
The size for a cover image is 1400 by 2000. It can't be smaller. If the embedded image won't upload for the cover, you have to do a new one.