If you are missing album artwork in iTunes or you want to create your own artwork, it's easy to updated the album artwork in iTunes to be whatever you like. You can either change the artwork for a single song, or for all songs on an album. To change it for a single song, select the song of interest from your library. If you want to change artwork for an entire album, sort your music by album and select all of the songs on the album. Once you have made your selection, "Get Info" on this song or songs (you can either right-click and select Get Info, or you can go to the File menu and select Get Info).

Once the info panel is opened, select the Artwork tab. If there is currently artwork assigned it will be shown. Just click on it and then select the Delete button. You can then select the Add... button and you'll get a file navigator. Navigate to the file that contains the artwork you want to use and once you are done and click OK, the artwork will be changed in iTunes.