Announcing the Cover Art Archive, MusicBrainz says:
So far, we’ve collected nearly 100,000 images that are attached to 54,000 releases for a 5% coverage in MusicBrainz. The largest file we have clocks in at 23MB and the largest image is 16,000 x 7842 (125 megapixels!). For all of the juicy stats on this project, check out our cover art statistics page.
This is a resource that programmers will be able to use to allow them to add auto-cover-art-downloading systems to their programs. MusicBrainz originally avoided having this sort of a system due to the legal implications of easy access to cover art (heaven forbid the music companies allow people to access the artwork for music), so I’m glad that they seem to have resolved it.