OS X 10.8 and later. This script will export then re-import the artwork data of the selected tracks obliging iTunes to embed the artwork in the track's file's meta-data.
For best results, use with an album's worth of tracks--or so--at a time. iTunes will 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: this app does not affect how the Finder may or may not display a file's icon image.
Also see Tracks Without Embedded Artwork.
Perhaps of Interest...
downloaded 8m 46s ago
Exports and then re-imports selected tracks' artwork
downloaded 12m 47s ago
Add or subtract a user-entered number from each selected track's Plays
downloaded 39m 26s ago
Set the Plays of selected tracks.
downloaded 47m 8s ago
Restore each selected track's artwork from an image file stored in its Album folder
downloaded 50m 8s ago
Display and export playlists' name, size, time, and track count for selected source
downloaded 51m 11s ago
Locates pairs of track entries pointing to the same file