It would be more comprehensive to add a few more cover art types (really, these aren't so much cover art as they are art generally):
For releases, specifically promos only, an art type of "Press release" would be useful for adding scans of the record label's sheet or two of promotional copy sent out to magazines etc.
Where it gets trickier, in a schema changing sort of way, would be if you wanted to also add non-release-specific artwork.
Release groups could have an art type of "Advert", for the adverts magazines and websites have for a particular album, which generally aren't format-specific.
Similarly, events could have an art type of "Flyer".
As a tentative, fanciful suggestion from a random user...