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 59s ago
Create a Ringtone track from a selected track
downloaded 3m 29s ago
Renumber order of selected tracks
downloaded 9m 18s ago
Exports and then re-imports selected tracks' artwork
downloaded 11m 45s ago
Correct song titles listed as "Artist - Song Name"
downloaded 14m 1s ago
Batch set new Last Played/Last Skipped of selected tracks
downloaded 18m 18s ago
Use tag data to formulate new file name for selected tracks' files