I think that by adding a "artwork size" column one could sort the music library in order to replace small or not rectangle artwork. I suggest that the display form should be: "width"x"hight"px (500x500px), and the sorting algoritm should be by the product of the two (smaller surface -> bigger surface), not by one or the other.
You can add that column yourself with this as value:
[%_cover_width% x %_cover_height%]
If you do not like the sorting, enter something of an $ifgreater() statement in the "Sort by" property:
$ifgreater(%_cover_width%,%_cover_height%,%_cover_width%,%_cover_height%)
Thank you, it worked great 
Where can I find a directory of all this operators: %_cover_width%, %_cover_height%, etc.?
Just to add a little more to this topic... like the OP, I like to know what has a cover, and the sizes, and the file size. I have three columns that tell me (1) if there is a cover and how many, (2) the dimensions, (3) the file size. I use these to make every track has a cover, only one cover, and that it meets my criteria in terms of dimensions & storage space.
See attached...
