Turn your computer on, open the iTunes program and access your music library. Minimize the window and log on to the Internet. Find and save the image you wish to reassign as the album art to your computer. You can do this by using Google images or finding the correct cover on http://www.amazon.com.
Reopen the iTunes window. Find the song or album you wish to use as your album art. If you wish to change an entire album's worth of cover art, you will need to change each song individually. Right click the song and select the option "Get info."
Select the "Artwork" tab. You will be able to see the cover art currently assigned to the song or album. To change it, select the option to delete the current cover art. Click "Add" to upload the new image from your computer. Browse through your computer's documents until you find the picture that you saved and select it. This will now be the cover art that shows up for the particular song.