Click on one of the songs and go to the Advanced menu and click on get Album Artwork
The songs that iTunes does not recognize wont get any album artwork though so you might still have a few left without it
The best way to go about it would be to make sure that these fields are filled out in itunes: Name, Artist, and Album. Once those fields are filled out then you go to itunes Advanced Menu and click on Get Album Artwork. That should obtain most if not all the artwork for your music.
The other way would be to manually add in the album art with pics from the web, or scanned images from the CD covers.