You won't be prevented from saving image files to the partition being imaged, but you should not do so. If you ignore this advice and save image files to the same partition as the one being imaged, you may very well end up with a partition image which has a file system in an inconsistent state. Another possibility under these circumstances is an imaging process that does not complete normally, but rather continues until no more disk space is left. These things can occur because the image files are being created at the same time the imaging process itself is carried out.
If you restore from an image which was created under these circumstances, be sure to run Scandisk or CHKDSK on the restored partition. If you are presented with an option regarding lost clusters, delete them.