First open your photo in Photoshop and duplicate it. (Just so at the end you can toggle and see the difference between start and finish.) For this example I am using tampadan's Mother and Daughter photo because of the strong yellow/green cast it has. Once the image is opened and duplicated go to Image - Adjustments - Threshold and move the slider all the way to the left until you are left with just a little blob of black about 5 pixels across, no smaller. Move the cursor over it (you will have defaulted to the eyedropper tool) and hold Shift and then click on the blob. This will set a marker on your image.Now go back to the Threshold slider and pull it all the way to the right until you are left with just a small blob of white 5 pixels across and Shift/click on it too. Again this will set a marker, and it will be numbered #2. When doing this pick make sure the white blob is not one made by a shiny highlight in your image. For example the glint off of glasses or any reflection off of metal is not good to use. (You can click on and off the Preview checkbox in the Threshold dialog to see what you are picking.) Cancel out of the Threshold dialog box. Don't worry about the markers disappearing, they'll come back.