The key is not unique across scans, but it is, hopefully unique to each track.  If two files both have artist "The Police" and album "Synchronicity" and and contain an image with 54321 bytes, I'm willing to assume that they have the same album art.  More foolproof would be to compute a checksum on the image. And if the Blob API had a native checksum function, I'd do that.  (Hmm.. maybe I should propose that)  But I this is good enough.