What product are you talking about? A backup is a generic term and an image is a type of backup. Another type of backup would be an incremental backup that only contained changes since the last backup. An example of an backup strategy would be to have your backup software do a weekly image backup and then do a daily incremental backup and you would keep a certain number of image backups in case you needed to restore a file (or the complete system) from an earlier point in time.