Check THIS website out. By far, it is the most substantial, comprehensive collection of album art I’ve ever found. And all really high quality too. Plus a nerdy little blog about the new cover art that comes out each week. It’s awesome.
I stumbled on it a few days ago and went on a rampage through my iTunes. The only albums that didn’t have cover art were a few John Mayer live shows I had, but everything else had at least some kind of art. The problem was that a lot of the cover art was from Google images and wasn’t the same size, good resolution, etc. So I went through all my cover art and replaced as many images as I could that weren’t uniform. And aside from the approximately 10 dissimilar album covers I couldn’t find, all of my albums now have consistent album art. It is a wonder to behold.