Hi Tim Blundell,
Thanks for posting in Microsoft answers!
System image is in essence a snapshot of an entire drive(s). The backup is done in block level (as opposed to file level) increments and includes all user and system files, configuration data and applications that are present on the drive, plus information regarding disk layout and boot entries. The image can be used to recover a working Windows if your hard disk ever fails, or if you simply want to re-image your OS to an earlier point in time.
System image is supported on internal/external disks, optical removable media, and network locations (Business edition or above).
You can create system image either on hard disk, external hard drive or on writable CD/DVDs. You could also save it on a network drive – which you have already done.
You can also include other drives (apart from the drive which contains Windows 7), but that would add on to the size of the final image. The number of DVDs required will be equal to the size of the partition that you are backing up.
Once you select the drives you want to back up, you get a confirmation informing you the amount of space required by the image. Depending on that the number of DVDs required would vary.
A progress meter is displayed while the images is created and backed up.
Refer the link below for more details on System Image backup:
Learn more about system image backuphttp://blogs.technet.com/filecab/archive/2009/10/31/learn-more-about-system-image-backup.aspx
Hope this helps!
Regards,
Gokul - Microsoft Support