i have been getting file sets in with several different covers for the same album. yes, it's very, very odd! anyway, the current artwork area doesn't allow removing the artwork in one fell swoop if the art is different in one of the files.
adding that option - clear all artwork - to the artwork field's context menu is something i would greatly enjoy.
this is using 2.38 & mp3 files. if i have 6 with the same art and two with NO art. there is no way to clear them all without UN-selecting the no-art files. the context menu just shows "add cover" with the other options grayed out.
this seems to apply whenever the "cover varies" notice shows up on the cover art area. are you using a different version of mp3tag or perhaps a different file format?