We currently have no easy mean to know if there is cover art for the release group as we do for the release. Not a perfect solution, ideas and patches are welcome. We should not query the json file at all if we know there is no cover art for this release group.
MusicBrainz Picard
MusicBrainz Picard is a cross-platform (Linux/Mac OS X/Windows) application written in Python and is the official MusicBrainz tagger.
Picard supports the majority of audio file formats, is capable of using audio fingerprints (AcoustIDs), performing CD lookups and disc ID submissions, and it has excellent Unicode support. Additionally, there are several plugins available that extend Picard's features.
When tagging files, Picard uses an album-oriented approach. This approach allows it to utilize the MusicBrainz data as effectively as possible and correctly tag your music. For more information, see the illustrated quick start guide to tagging.
Picard is named after Captain Jean-Luc Picard from the TV series Star Trek: The Next Generation.
Binary downloads are available here.
To submit bugs or improvements, please use Picard bug tracker.