The only way to get artwork for your files in iTunes is to buy the song. If you (like most people) import a bunch of mp3's into iTunes, then you will come to know the joys of tag management. here's what you do.
1. Import all your MP3's into iTunes.
2. In the iTunes Library, sort all your songs by album.
3. Fire up your browser and go to Amazon.com.
4. Now the fun begins. Look up each album in Amazon. For each album do the following:
4a. right-click on the album art in amazon and select "copy".
4b. switch back to iTunes and select ALL the songs in the album.
4c. right-click on the album art area in the lower left corner and select "paste".
4d. if iTunes asks if you want to update multiple tags at once, select "yes".
5. Repeat step 4 for each album.
This will (obviously) take some time. Once you've finished doing this, however, you'll have nice album art for all your mp3's. I set up a smart playlist that shows me songs added within a certain period. Every time I add new mp3's to my collection, I use that playlist to manage my iTunes tags.
I did see a program that hunted down album art and populated it automatically in iTunes, but I've never been able to find it after I wiped my machine and re-built it.
Musicmatch Jukebox will tag songs automatically but their album art portion doesn't always transfer to iTunes. and you have to pay to gain access to their database of song info. Amazon will probably have 95% of the album art you need.