albumartexchange is in fact my first priority selection because of their quality scans.
While different CD cases have different sizes, the site's scans are also not consistently sized,
although most of them are (nearly) 600x600.
Perhaps I am too obsessed with uniformity? Does size matter?
In the end I think I would be nice to have MusicBee run through and update all missing artwork
and for everything not fitting, I am left to either choose a different format or scan/stretch the artwork myself.
Right now I have about 2000 albums of which most of them are still missing a quality jpg.
(oh well, everything was easier with my mp3 collection before, when I did not care...)
To round this off: I would welcome a filter before changing my attitude... Thanks for asking