I promised to pass on details of my triumphant and successful technique for fixing the artwork in the iTunes library, and on the iDevices you download it on. Unfortunately, although I believe I can fix problems in the library (On the Mac / PC), there are some songs where the artwork will not update on my iPhone. Even deleting and re-downloading the file only brings the old artwork back. I am only writing about this here as I know I am not alone in having this problem.
The songs concerned are all ones I imported with embedded (Poor quality) artwork. They have all subsequently been matched so the files downloaded to the iPhone are shiny new aac files from Apple and have no association, apart from the matching process, with my old files. Somewhere, in the iTunes match universe, they are being reattached to the artwork my original files had when I first imported them. My current efforts are being directed towards understanding this part of the process, and I will let you know when I have a reliable way to bring it to heel.