using Preview to alter, then save both versions, of a photo

I take photos in courtrooms, with judges' permission.

Sometimes they order me to "tilize," or obscure the face of a person -- the defendant, or a witness.

How can I import a photo using Preview, and use a simple tool to hide a face, and then save BOTH versions of that photo?

(Because after the case is finished, the judge will often "lift" the tilize order -- for instance, if the defendant is found guilty and sentenced.)

Thanks for any help, Eva