[gruez]

>A “watermark” is simply a version of that designed to be invisible or imperceptible to the human eye, for example by embedding information in the least significant bits of the pixel colors.

In this case however, it's hard to imagine how you'd do this without being detected. If you loaded the screenshot into mspaint and used the paint bucket tool, any color differences will become apparent. You could limit your modification to the edges, but detail there is going to get wiped by jpeg compression (if any).