Changing album covers after the fact

Great suggestions. I will devote a snowy, frigid day to the task. Thanks to all. 

To add to rhanson739's suggestion, a good source for album artwork is amazon.com's "CDs & Vinyl" section. Search for and open the album page you're interested in, click on the "Click to open expanded view" option under the artwork. Right click or Control click on the artwork and choose "Save image as." Save it to your desktop and drag it into the "Artwork" tab rhanson739 mentioned.

If you're storing files in iTunes, it's simple. Go to the album, hit CMD-I (on a Mac), and select the tab for Artwork.  Drag a JPG into the box, and it will replace the cover for the entire album.

Lots of software will let you do this ... MP3Tag, Winamp, JRiver Media Center ...