Find the image you want online, and save it to a folder on your desktop.
Open iTunes and select the song or songs you want to use the art work for.
Right-click on the song. Click "Get Info", and go to the "Artwork" tab.
Click and drag the album artwork from the folder to the "Artwork" tab.
To edit album artwork for an entire album, click on the "Album" column on the top of the music library. Click the first song on the album, then hold "Shift" and click the last song in the album to highlight the entire group. Right-click "Get Info," the "Yes' when iTunes asks if you want to apply this action to multiple items.