UPDATED: Restore Artwork from Album Folder v2.0
Restore Artwork from Album Folder v2.0 will look for an image file in the parent folder of each selected track's file--presumably its "Album" folder--and apply the data from that image file as artwork for the track.
Requires iTunes 10.3 or better. This latest version is a Cocoa-AppleScript applet for OS X 10.6 and 10.7 only. The previous version is available at the script's entry page.