For a detailed description of what a disk image is click here.
To initiate the backup wizard choose one of the following options:
Choose 'Backup' then 'Create Image' from the main menu.
Select 'Create a backup image' from the 'Backup Tasks' panel.
Right click the partition you want to backup in the drive layout window.
Right click in Windows Explorer and select 'Create an image of this partition...'
Next:
Create Backup Wizard