Someone has stolen your picture but that person says that's not yours. She has deleted your watermark and all the metadata of the file or she did a printscreen of your picture. How can you still prove it's yours? Put something in your picture that only you know it's there... like a pixel.
Here is what I do. Open your image in Photoshop and zoom in at the pixel level. Choose a pixel, preferably in a place that you will remember, and turn it completely white using the eraser or the brush tool. Open the color picker tool and choose a color. Write down the RGB values. With a hard brush set at 1 px, paint that pixel in the chosen color. You might have to apply the brush a few times so that the pixel turns exactly the color you chose. Make sure not to paint pixels around it.
The RGB values become your unique identifier number for that pixel if someone tries to reproduce the same color. Needless to say that you should try to add your color pixel always at the same place, for example, 50 px from the top and 25 px from the right. You can even use the RGB values as position values of your pixel.
This is not of course a full proof solution but it certainly doesn't do any harm to have that extra pixel as a 'fingerprint' on your image. And by the way, no, you can't see the pixel even though you know it's there. I have done the test with lots of people and nobody noticed.