Did you create this image yourself? If so, did you choose to image only a partition or the entire disk?
Did you create the image shortly after installing the operating system?
Just so you know, because a disk size is 640GB does NOT mean the image has to be 640GB too. The only thing that the image will store is actual data, not the absence thereof. Not to mention the image will be compressed, so may appear smaller than the amount of data you expect to be stored in it.
If you are worried about losing documents or data on the computer, and you're not sure what all is in the image you are about to flash, it maybe worth making a backup of your HDD before proceeding (it's probably a good idea regardless.)