The Cover Art Archive is a joint project between the Internet Archive and MusicBrainz, whose goal is to make cover art images available to everyone on the Internet in an organised and convenient way.
Images in the archive are curated by the MusicBrainz community and go through a peer review process to ensure that they are correct, free of spam and of the best quality. If you would like to contribute cover art, create a MusicBrainz account and follow the guide.
MusicBrainz also provides API documentation for retrieving cover art via a release MBID.
If you have any questions regarding the images in coverartarchive.org, please email info [at] archive [dot] org.
For a slideshow of 9000 of the covers, download the files from here (warning it is a 9GB download).