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.
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