http://musicbrainz.org/edit/17498648 (add cover art) has the types on new lines. http://musicbrainz.org/edit/17469323 (edit cover art) has the types on one line separated by commas. http://musicbrainz.org/edit/17487464 (remove cover art) has the types on new lines.
I prefer them on one line separated by commas, but whatever we decided to do we should at least be consistent.