Target partition to small for image backup

I'm using only 70 GB of the 883 GB (C): partition on my hard drive. I shrunk the partition by 100 GB to create a
new 100 GB partition for an image backup of the 70 GB of system and data files. Yes, I realize that an external drive would be best, but I don't have one right now. Anyway, Win 7 image backup claims that my target drive is to small for the operation (after xferring about 20 GB to the new partition). Does the backup software not realize that 70 < 100 or is there is more to this than I realize ?

Well. I did use the Windows default sources which included D: and C: , but that only added another 11 GB which is still considerably less than the 100 GB.

Can you post a screenshot of Disk Management?
Disk Management - Post a Screen Capture Image

What do you mean by "(after xferring about 20 GB to the new partition)" in post #1 ?
Did you xfer 20 GB to the new partition, or did the Win 7 Backup program create 20 GB on the new partition?

You could also try a different Imaging Program such as Macrium (free).
Imaging with free Macrium
A link to download Macrium is in the tutorial.

The Windows 7 backup program begin the image save to the newly created 100 GB partition without reporting any error condition. The save operation persisted for several minutes until about 20 GB had been saved to the new partition when it aborted and reported the lack of sufficient space message.