This script will look for an image file in the parent folder of each selected track's file--presumably its Album folder in which you have previously placed an image file of some sort--and apply the data from that image file as artwork for the track.
OS X 10.6 or later only. iTunes 10.3 or later only. Previous universal version available at the link below in the "Info" section.
Remove iTunes tracks disassociated from files
Copy Artist folder, Album folder and file names to appropriate tags
Applet assists with swapping, copying, appending data between track tags
Perform search-and-replace on text in your choice of tags
Assist with discovering and eliminating same-named tracks
Delete and trash tracks from iTunes whose play counts are 0