This script will embed artwork that has already been downloaded (via the iTunes "Get Album Artwork" command) into the files of the selected tracks.
Also see Re-Embed Artwork.
NOTE: For best results, use with an album's worth of tracks--or so--at a time. iTunes may be obliged to perform a number of background tasks when applying image data and may prevent the script from proceeding if a large amount of data is involved.
Note: (August 2014) Not sure what the issue is but results are inconsistent with modern versions of iTunes.
Perhaps Also of Interest...
downloaded 10m 29s ago
Export files of tracks in two or more playlists to single folder
downloaded 17m 48s ago
Use tag data to formulate new file name for selected tracks' files
downloaded 30m 14s ago
Delete characters from the beginning or ending of selected tracks' name, artist, album, comments, composer, or show tags
downloaded 32m 4s ago
Displays files in iTunes Media folder not in iTunes track library
downloaded 32m 41s ago
Drag-and-drop to convert audio files with iTunes
downloaded 42m 7s ago
Append user-entered text to beginning or ending of selected tag of each selected track