*Why is artwork not showing on my iPod when it's there in iTunes?*
iTunes will display a cover in many of it's views provided at least one track from the album has embedded artwork. The iPod appears to load only the artwork from track one to create the cover flow and album views which means that it can sometimes fail to display art for albums even when you can see the art in iTunes. Further, when tracks are played on the iPod, artwork is displayed if and only if it exists in the currently playing track, so track one may show art while track two does not.
*How to find tracks without artwork*
To find all the tracks without artwork so that you can update them you can try
http://www.stum.de/itunes-find-tracks-without-artwork/ for Windows or
http://dougscripts.com/itunes/scripts/scripts13.php for Macs. Use Google, Amazon, Discogs etc. to locate relevant jpeg images. Ideally these should be square, 320x320 pixels or above and borderless to give the best results in the various menus. If you have artwork for some tracks of an album, but not others, find the album in the main music folder, select a track with artwork, right-click (Option-Click for Macs) on the art & click copy, then select all the tracks of the album, use CTRL-I or Command-I to *Get Info* and then paste the image into the artwork box.
I say 320x320 because I believe (from using TouchCopy) that it's the size used in the iPod cache so if you're going to crop or resize you might as well work to that size. Otherwise 200x200 is probably good enough.
*Rebuild artwork cache*
Sometimes the artwork cache becomes corrupt and fails to show correct artwork, even when this is properly set up in iTunes. To rebuild the artwork cache, connect your iPod to iTunes. Locate the iPod in the Sources pane on the left-hand side, then select the Music tab. Remove the tick from *Display album artwork on your iPod*, sync the iPod, reselect the option & sync again.
tt2
