Did you know that you can have multiple pictures assigned to a song's artwork tag?
Why would you do this? Example: Jane's Addiction "Strays". The limited edition of this album that I got came in a slipcase. The slipcase is therefore the front cover of this particular CD. I scanned this and tagged it as my artwork. I then scanned the CD booklet, which is also the cover of the standard edition, and added that as well. Now I can see both the limited and standard edition artwork.
To do this is simple. Firstly, select all the tracks to which you want to assign a piece of artwork. Drag the artwork to the lower-left artwork pane of iTunes (you may have to show it using Edit/Show Artwork or Apple-G). Then drag the second or subsequent artwork onto the artwork pane. iTunes will add each new artwork to the songs. You can scroll through them by clicking the arrow buttons that appear at the top of the artwork pane or by getting info and clicking the Artwork tab.
When you sync with the iPod, all this artwork is saved along with the songs, even though you can't see them.