How do you change the album art on itunes once there's already a picture for that song???

because when i try put another picture to replace it it changes for a second and then goes back to original pic. ( im on a mac by the way)How do you change the album art on itunes once there's already a picture for that song???Open iTunes then left click on the song you want to change the album picture then click on Get Info then on Artwork from there you can add or delete.