Yes, you can save a system image on the same computer as long as it's saved to a partition or HDD that is not included in the system image, and has enough free space to hold it. It will not let you save it to one included in the image.
However, I wouldn't recommend to bother with transferring the system image backup, but instead just create a new one and save it directly to the external HDD. Afterwards, you could delete the one on your computer if you like. When you move a backup, it tends to get corrupted easy and possibly be of no use to be able to restore from.
Hope this helps,