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. To see what has been added to the archive recently, please see the Cover Art Archive page at the Internet Archive.
MusicBrainz also provides API documentation for retrieving cover art via a release MBID. A C based client library libcoverart is available as well as the Java coverartarchive-api library.