That isn't watermarking, wap, though that is what most people think of it as. What you are referring to is what people typically do in order to make a comp or preview image unusable without purchase...you'd have to go in with a clone stamp and clean out the branding overlaying the image...not fun.
A true watermark is digital information that has been stored in the binary data of the image, containing copyright information. This data survives numerous edits of the same image, survives being copy and pasted (to an extent, for a few iterations) and is the only true way to give graphical copyright infringement claims any merit or teeth in a accusation or deliberation.
The link mouse gave you is exactly what you should look into.