I know that this thread is almost dead but I have to add this information:
Originally after getting good advice herein on how to upgrade the iTunes artwork I went to work upgrading everything using Amazon Music. Then, as a back-up plan, I checked out seekacover.com. Well, let me tell you, the difference between Amazon and seekacover is like night and day. I just finished re-doing all of the artwork using click and drag from seekacover. Their stuff is first class quality.
Just wanted to share that with any readers that may still be checking in.