Click "Library" and select the album for which you want to download artwork.
Right-click the album title and navigate to the option "Find Album Info."
View the artwork. If a music note icon or generic image appears instead of your artwork, that particular album artwork is not available in the database.
Click "Preferences."
Choose the option "Store."
Click "Automatically Download Missing Album Art" and check the box next to it. This will allow iTunes to search for album artwork that may not have automatically downloaded when your music did. If the album is not available in the iTunes store, the artwork will not be downloaded.