You will find that sometimes the album art for your music is missing or incorrect. You can manually add an image to fix this.
1. Open iTunes and go to your Music Library. View it in Grid View so that all of your albums are displayed.
2. Right-click the album whose album art you wish to change. Select Get Info.
3. When asked if you are sure you want to edit information for multiple items, click the Yes button.
4. Go to the Artwork section and double-click on it.
5. Browse to the desired album art and click the Open button.
6. The songs will be updated with the new album art and it will then be displayed in your Music Library.