Q: artwork

Well, if you've upgraded to iTunes 7, the easiest way is to go to Preferences (found in the iTunes menu). The fourth checkbox from the bottom should read "Automatically download missing album artwork". If you check it, you'll get a message letting you know that information will be sent to Apple (namely the identity of album tracks that don't have artwork). Click "Yes" and you're Mac goes to work. Additionally, if you download a new album (and are online), iTunes will automatically check and add the artwork for that album as well.

However...

This system apparently uses the iTunes Music Store for it's information. So if you have albums that aren't available through iTMS, those tracks won't be affected. It also uses the artwork shown in the iTMS, which may differ from the art you are expecting if it's a foreign release or re-release.

If iTunes can't find the corresponding artwork (or you just prefer a different picture), it's a little harder. Highlight the tracks you want to affect and choose Get Info from the Edit menu. If you're editing a single track, click on the Artwork tab. Otherwise, you should see a square on the right side of the Multiple Item Information window. Now if you can find a copy of the album artwork, simply drag it from your browser or other program into the "image well" and click OK. I tend to use Amazon.com to find cover art, but I've heard some people swear by Google. You can even use a scanner a digital camera in a pinch!

Hope that helps!