try a very useful little programme called Clutter... it's freeware and will show the music being played by iTunes in a little window...
it allows you to search for artwork, choose which is correct and - when you send it to iTunes - applies it to all tracks for the album you're playing a track from...
beats having iTunes apply the same artwork to every album that has "Greatest" in the title, for instance...
if Clutter can't find the artwork, you can google the album name and, if you manage to find the cover, save it to the desktop, play a song from the album, than drag the image into the "Currently Playing" window in iTunes... it'll apply the image to all tracks on the album