I am using Windows 8 with Calibre 1.0.0.
I have a book "Gardens of the Moon" that I gave a new book cover to with a dimension of 790 x 1200:
Afterwards, I used "save to disk" and subsequently re-added the file that was output into my library. However, after the book was re-added into my library, the book cover's dimensions were changed to 671 x 1020 and looked very pixelated/distorted:
Any thoughts what's going on?