Over the original image, create a new adjustment layer. Make a selection that goes around the edges of the teeth with the Lasso tool. It’s fine if some part of the lips or gums are captured. Select the Hue/Saturation adjustment layer from the adjustments drop-down menu. It will change your selection into a layer mask automatically. Click the choices bar labeled Master in the Properties panel of your Hue/Saturation layer and pick Yellows, since that’s all you’ll need to whiten your teeth. Move the saturation slider to the left to reduce the yellows in your selection. If you see a rough edge to your selection, select Masks in the Properties tab to correct it. Increase the feather slider in the Masks panel to give your pick a softer edge.