Backup confusion of a rookie

I just made an image backup of my system (Win-7, C/:) on an external USB drive.
The Win-7 program indicated there are 279GB on C/: and the image file will use 33GB.
The finished file was indicated as only 20GB...and although I am the Admin and sole user of the computer, I was unable access and view the files??

Later, I read that I can simultaneously make a backup of all my files _and_ an image file of Win-7 in a single operation. Then, why wouldn't one check 'make Win-7 image' and backup their files initially, then on later perform personal file updates backups and leave the 'make Win-7 image' unchecked??

Btw, I've yet to set up the Win-7 backup program...as I'll probably use some other faster software for imaging and file backups.

thanks for any help...