When doing image backup, it will take the full image of the selected drive at first time. In the successive backup schedules, it will compare the previous backup content and the current data to find the difference int he drive since the last backup. Then, it will find out the changed blocks and create incremental image for that difference data. Because of this, it will take more time than the initial full backup.
If you want to take backup of files (not the entire disk as image) or disk image, try the StoreGrid backup software.