Originally Posted by C8H10N4O2
I agree. I go with the largest cover I can find and then shrink it down if I need. Sizing them up creates a lot of artifacts.
If you are making a cover quality plugin could you look into a cover download plugin that gets covers from google images instead of google books. With ebooks having gone mainstream usually the first 5 results for a cover by book name using search the internet plugin results in very high quality covers. So the ability to use google images from inside calibre would be really great.