No doubt Album Art gives a better view while listening to the songs both in iTunes and iPhone but if you have saved hundreds of songs in your iPhone without the album arts and are wondering how to add Album Art back to these songs, then below are the steps.
1. First make sure you have all the images of the songs that you wish to put as an Album Art.
2. Now follow the steps in this post that will guide you to add the images actually to the songs.
3. To backup the songs stored in iTunes library, run iTunes and right click on Music and then Export.
4. Save the list as text file say “music.txt”.
NOTE: Do no move any song from its location on the hard disk else it will not get restored.
5. Now select all of the songs in the iTunes library and delete them. This will remove all the songs with no album art.
6. To restore the songs with Album Art, click on File -> Library -> Import Playlist and browse for the music.txt file. This will restore all the songs with the album art.
7. When you sync your iPhone for the next time, you will see the album art on the songs.
Believe me it is very simple.