Also - to get a (usually) bigger image from Amazon, and without the "look inside" or "kindle" image decals:
On the Amazon book page, right click (or ctrl+click for Mac SBM) and "open image in new tab". Go to the new tab, and you will see this:
In the browser address bar, delete everything after the first "." up to the jpg extension like in the example below (part to be deleted is bold):
which, once you hit enter, will load this:
Then you can either save the image or copy/paste to Calibre. Very helpful when large enough images can't be found elsewhere. Edited to add: The reason I mention this method is because those covers on Amazon with the zoom option no longer copy/paste properly most of the time because of the overlay.