>on windows quick & dirty:
-goto your rom-folder and type this command:
for /f %x in ('dir /b /on') do @echo %~nx >> C:\roms.txt
-goto your artwork-folder and type this command:
for /f %x in ('dir /b /on') do @echo %~nx >> C:\artwork.txt
-type this command and look at differences (this are your missing artworks):
fc roms.txt artwork.txt
(or use freeware tool
winmerge )