If none of your artwork appears on your iPod, make sure you are using iTunes 4.7 or later and make sure you have selected the Display album artwork on your iPod checkbox in the Music section of the iPod pane in iTunes Preferences.
If you manually add some songs to an iPod but then change iPod update preferences to automatic in iTunes, the artwork for the manually added songs will not show up on the iPod.
Solution
First, switch back to manual updating of the iPod:
Then delete the songs that you first added to the iPod manually:
Then Switch back to automatic updating of the iPod:
For information on adding artwork to songs in iTunes see iTunes Help or click here.
This document will be updated as more information becomes available.