For Windows: iCoverArt

http://www.maximized.com/shareware/icoverart/index.htm

For Mac OS: AppleScript to run while running iTunes.

Code:
tell application "iTunes"

   repeat with aTrack in selection

      repeat with anArtwork in artworks of aTrack

         if downloaded of anArtwork is true then

            set theData to data of anArtwork

            set data of anArtwork to theData

         end if

      end repeat

   end repeat

end tell
_________________________
Bruno
Twisted Melon : Fine Mac OS Software