I use a small stamp made out of the round head of a steel bolt. I etched my signature in it's surface with a dremel. By a light blow with a heavy hammer I stamp an imprint in the image area itself. Mostly at an unobtrusive spot in the lower left or right corner. The mark it leaves is just a fraction of a millimeter deep, and can only be seen under oblique lighting. It really doesn't interfere with the image. But it's absolutely indelible, and nobody has the same stamp.