I use a bunch of iTunes art searchers/importers but as far as convenience goes, this beats them all. Just select the songs (i.e. all songs in the album) in iTunes, open up the widget and let it search, then click again to add the artwork. This method is great for quickly adding artwork to music you've just imported, though it could get a bit tedious if you already have a large collection and if so you'd probably be better to use something like Coverscout.
I highly recommend AAAW :)