Some publishers have high resolution covers available for download on their website. The
Tor/Forge website for example. I like to use the publiaher's cover if possible, as the Amazon/Google covers have often been resized with a different aspect ratio to the original.
In Calibre I have the following tweak set:
Code:
maximum_cover_size = (758, 1024)
so when I add a large cover to the book it is automatically scaled down to suit my Glo's screen. I also have the "upload covers for books" option ticked in the KoboTouch driver, so Calibre sends the covers instead of letting the device generate them.