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.
Changing album covers after the fact
I've ripped my entire cd library using dppoweramp. When using my iPad as my remote, I've noticed that some of the album covers displayed on my iPad are some alternate cover and not the actual one I have. (i don't know if I just screwed up or the software over rode what I selected.). Not a big deal but is there a way to easily change album covers without re-ripping the cd?