This script will apply a [no cover] graphic as the artwork to the selected iTunes tracks.
Also included is a droplet that will enable you to convert your own artwork as the default artwork used by the script.
(Note: In order to launch this PowerPC-era script on an Intel machine, it must first be opened and then saved using Script Editor (AppleScript Editor pre-OS X 10.0) which is located in your /Applications/Utilities/ folder. More information is here.)
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