-Make a new layer on top of your final picture, then select the rectangular marquee tool,
-Then (what I do) zoom too 200%,
-Make sure that the left of the marquee "cross" is touching the left side of the sig, and the bottom of the "cross" is on the bottom of the sig, then click and drag to the top right so that the top of the cross is touching the top part of the sig, and so the right side of the "cross" is touching the right side of the sig.
-Then right click in the middle of your sig, click "Select Inverse"
-Then right click on the selected area > Stroke.
-Then (what I do) 1 pixel, center, color black, set it to overlay, click "ok", and you're finished.
Hope this helps.