That was a forum question, which I answered as follows:
It is, indeed, a mystery, and, apparently the writers of Amazon help files don't know either, because different answers are given in different places.
The Kindle does "reformat" photos if they aren't the right size... that's why you get distorted or too-small photos even when you upload a large photo.
But what is the right size is the question.
I don't know, but I can give you some that work to make a full size image for the cover page:
600 x 800 px given in Amazon Instructions for HTML use.
1200 px longest side Amazon Instructions in messages and at the Dashboard. IF you also get the other side right.
The easiest way I've found to make a good cover image is to take a *portrait* mode photo that's 1600 x 1200 straight out of my camera, and reduce it by 75%. (It has to be rotated to make it a portrait mode to fit the Kindle cover)
That gives a 900 wide by 1200 high image, and makes a very nice cover image.