If you have an iTunes store account, you can use the "get album artwork" option in iTunes. You don't actually need to purchase anything from the iTunes store, just to have or create an account. Provided that the album or song for which you are looking to add artwork is available for sale in the store, iTunes will find it and add it to your library.
If you go to preferences/general in iTunes and check the box marked "automatically download missing album artwork", it will automatically add the artwork when you import a CD into iTunes.
One downside to this is that if you have an album or song misspelled in your iTunes library, it may not be found by iTunes because it will be looking for a match.
See:
How to add artwork using iTunes.
If you don't have an account or don't want to create one, here are a couple of other methods.
First off, make sure you have "show album art" enabled in iPod options.
Highlight a song or album in iTunes, then right click on it and select the "get album artwork" option.
Go to
Google Image Search and type in the artist/album name and click on search images.
Double click on a picture you like, then at the top of that page click on "see full size image".
When you get that image, simply copy/paste it into the space for album artwork in iTunes.
Make sure you select the album in the browser of iTunes before you copy/paste.
You'd be surprised what Google Images can bring up. There are literally millions, and even obscure album artwork can be found.
In the unlikely event that Google can’t find an image, you can also get artwork from Amazon.
If you’re happy using images from Amazon, and don’t want to copy/paste, then
iTunes Art Importer is a nice free program to do it for you.