These instructions will have to be modified if you've moved the iTunes folder from its usual location.
Back up all data before proceeding.
Triple-click anywhere in the line below on this page to select it:
~/Music/iTunes/Album Artwork/Cache
Right-click or control-click the line and select
Services ▹ Reveal in Finder (or just Reveal)
from the contextual menu.* A folder should open with a subfolder named "Cache" selected. Quit iTunes if it's running. Move the selected folder to the Trash. Relaunch the application and re-download all album artwork.
*If you don't see the contextual menu item, copy the selected text to the Clipboard by pressing the key combination command-C. In the Finder, select
Go ▹ Go to Folder...
from the menu bar and paste into the box that opens by pressing command-V. You won't see what you pasted because a line break is included. Press return.