To add album cover artwork to albums that iTunes didn’t download, you need to find the album cover image online somewhere. The best bets to find good images are the band’s website, its record label’s website, or Amazon.com.
When you’ve found the image you want, download it to your computer.
Next, in iTunes, use the regular view of your library and find the album you want to add artwork to.
Select all the songs in that album. To do this on a Mac, use Apple key + I. On a PC, use Control + I. Or, on either, go to the File menu and choose "Get Info."
Doing that will pop up this window.
