[rtiangha]

In terms of covers, there's a plugin called "Kindle hi-res covers" that will poll Amazon's servers for the highest resolution covers they have for that book that they push out for their higher resolution devices. They're usually supplied by the publisher so the quality is good most times. And if you want, you can use Calibre's Polish function to replace the cover image in the epub, but it'll increase the file size because of the larger file.

Another source for higher resolution covers might be the publisher's website and going through the book's gallery or preview options and see if you can save the jpg.