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 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: 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 Also of Interest...
downloaded 1m 54s ago
Two scripts copy text from clipboard to current or single selected track's lyrics tag
downloaded 8m 44s ago
Display, create text file listing info of dead tracks
downloaded 39m 6s ago
Copy Artist to Album Artist of selected tracks
downloaded 58m 20s ago
Gather audio tracks whose files contain no artwork metadata
downloaded 1h 1m 30s ago
Rename selected tracks' Song Names with their filenames (minus extension)
downloaded 1h 29m 13s ago
Sort purchased tracks into discrete playlists by name or Apple ID