See related ticket
I would be really useful if we had a way to store and make available release-group cover art (may be only for displaying on artist page).
My proposal is not to store yet another image, but rather have the release-group cover references one of the release front cover art.
A specific editor would allow picking the right cover art from those available (something like http://www.discogs.com/update/images?release=3848546 with some drag and drop between available images zone and another zone).
Release-group cover art would be initialized with some criteria (e.g. with the front cover of the earliest release having cover art).
Then a trigger would make sure release-group cover art is updated when a front covers are added/removed on related releases.
Ideally we would use http://coverartarchive.org/release-group/MBID