It's quite strange. If you taken the image of the partition (instead of the disk) you could always restore it as a partition. As you said, the target partition is 20GB so the image containing 12GB of data should fit.
1. Could you confirm that you were taking off the image of the source partition, not disk?
2. In GhostCast server, after you selected the image to restore, set the radio button to Partition and click on the drop-down list. Can you see the desired partition?