Well good news for you because this is a guide to show you how to add those images.
here is a screen shot of my library on itunes.
Step one: Find what the album or the songs name and google it (using "Just like a pill- P!nk")
Step two: copy the image to your clip board
Step three: right click on the file name on i Tunes and click "Get Info" which should be your second choice, and then click on "Artwork"
Step four: Now then all you have to do is press "Ctrl v" or right click and press "Paste", and the "Ok"
TA-DA and you 100% done you can keep on doing this to your other songs exc