iTunes cover art is of much higher quality than Amazon's because it is more predictable and, I guess, it comes directly from official music distributors.

If an iTunes link is added as relationship to a release, as I just did on https://musicbrainz.org/release/b142b0cc-6e97-4d29-b8ca-af9d5657ece5 , the MB server would have to scrap the iTunes page and find the cover art image URL, which would be:

http://is1.mzstatic.com/image/thumb/Music1/v4/27/b2/e6/27b2e6f6-6b9b-114c-337b-5f0862b65da1/source/170x170bb.jpg

This is a lowres 170×170 cover art image. But, if you change the last part of the URL for a deliberately big number, such as "3000x3000.jpg", you'll get the best resolution image, probably the highest resolution that the music distributor sent to Apple. In this case you'll get a 2400×2400 jpeg of 2MB size. Try it:

http://is1.mzstatic.com/image/thumb/Music1/v4/27/b2/e6/27b2e6f6-6b9b-114c-337b-5f0862b65da1/source/3000x3000.jpg

This is very predictable and never fails so I think it would be easy to implement on musicbrainz.org and very useful for the community.

Thank you in advance