Use Watson's Amazon tool to import album cover art into iTunes 4
Here's a hint by Zane Edwards I discovered on a recent MacOSXHints article. You can use the Amazon.com tool on Watson to save album cover art on iTunes 4. Just drag the preview from Watson into the cover art area for the selected songs.
One caveat: Be sure that the magnifying glass is active, meaning that the large preview image is available. Otherwise, the image that it imports will be a tiny placeholder graphic, which you don't want!
Update: Be careful about file bloat! Some sleuthing by Josh Rafofsky has revealed that various sources of image files result in drastically different file sizes resulting, depending on where you drag the image from. Even though the images are always converted to PNG, it seems that the images are stored differently depending on how the image was received. A drag from a file seems to result in a much smaller size than a drag from a program like Watson or Safari. I consider this to be a bug with iTunes (suitable for reporting to Apple here (if you're ADC member) or here (if you're not). So be careful about dragging from Watson, or other sources, if you are concerned about disk space!