This script will re-embed artwork downloaded from the iTunes Store into the files of the selected tracks.
Also see Re-Embed Artwork.
NOTE: this script cannot handle processing more than about an album's worth of tracks per launch due to the way iTunes manages the graphics data.
Note: (August 2014) Not sure what the issue is but results are inconsistent with modern versions of iTunes.
(Note: In order to launch this PowerPC-era script on an Intel machine, it must first be opened and then saved using AppleScript Editor, which is located in your /Applications/Utilities/ folder. More information is here.)
Display, create text file listing info of dead tracks
Drag-and-drop to convert audio files with iTunes
Enter text for the selected tracks' Long Description tag
Remove iTunes tracks disassociated from files
Append user-entered text to beginning or ending of selected tag of each selected track
Applet assists with swapping, copying, appending data between track tags