But you are correct with your assessment that AllCdCovers isn't by any stretch a premiere CD Cover archive. You do however, have the option to use Last.fm as your source for metadata and album artwork.
A good source for cover art images would be the difficult part for any of these programs that try to fix tags and add cover images. Apple has done well in getting a good cover art collection for iTunes. But that's Apple's toy and not something shared with other companies. AMG (AllMusicGuide) is a popular source. A well rounded collection that includes classical and jazz. But often the images are in the 300x300 range. I had a subscription to the AMG data through dBpoweramp. It got me basic cover art images when I was ripping my CD collection. But I've pretty much replaced every image from them since with larger better images, some of which I scanned myself. Better than nothing though.