Simple, you make a new document, and select as background: transparence, then you set the size to the same size as your render, in your example, the warrior. Than you use the box select tool and look above on mode, there you select fixed weight or so. On the right, you see there 2 boxes, there fill in as an example: 350px 120px. Then you click with the mouse on your workground and place the box there, were you want to have your normal sig placed. After you painted the normal sig in the box, you place your render in front of the sig and save it then as a .png. Hopefully you understand it, if not, just ask.