I've added quite a bit of album art—all of it scanned myself—mostly to classical releases. I have 901 add cover art with 0 votes, 2 with only no votes (both on the same release, and canceled), and 248 with at least one yes vote. IOW, about a fifth of my add cover art edits attract a vote.
So requiring that at least one review be obtained would be problematic.
OTOH, since cover art is effectively applied immediately (even though the vote is still open), I think it'd be fine to change the 7-day approval period to something longer. That'd give more time for anyone to object—and one person voting no is all it takes to make the edit be rejected, correct? (Provided he/she is not outvoted, of course).
And if even after two weeks, or however long, not a single person has stopped by to check—then I think our actual problem is a lack of people who care about that artist/label/etc.
(Another thing: it'd be nice if it were easier to move cover art to a related release).
[I've certainly run in to some annoying cover art uploads, mainly of annoyingly tiny variety].