OS X 10.8 or later. 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.
Perhaps Also of Interest...
downloaded 2m 2s ago
Batch set new Last Played/Last Skipped of selected tracks
downloaded 9m 58s ago
Batch merge and/or delete playlists
downloaded 12m 25s ago
View/Edit tags of selected track(s) in single window
downloaded 12m 47s ago
Applet assists with swapping, copying, appending data between track tags
downloaded 16m 8s ago
Renumber order of selected tracks
downloaded 16m 27s ago
Locates pairs of track entries pointing to the same file