here is an idea that might work.
make a playlist with your audio files, Start with only a few. make a column>add expression call it missing image
in the expression type
if(IsMissing([Image File]),Yes,No)
that seems to work for me.
or make a pane in a view with this expression
remember that the ismissing expression can take a long time before it checked everything (it has to look on the disks for every file), so the view or playlist can take a long time to load. you might want to cut the list in pieces with a big library, so that is checks only a limited amount of files. especially if you want to test it first
gab
edit: in your case, with the large library, you could make a smartlist with the collumn as described above, and, presuming that the problem will be the same for every song of an album, make the smart list select only track number 1, and maybe only artists starting with one letter. because im pretty sure that with your library it will take far to long to check all those files in one run. and maybe this will still be to big.
edit2: first make a backup of the library.