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.
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Create two playlists containing tracks with JPEG and PNG artwork
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Export artwork of selected tracks to parent or specified folder