Here is a simple tip on how to whiten teeth in a photo using Photoshop:
1. Open your file.
2. Duplicate. On your layers window, right click the background and choose duplicate layer. Tip: Duplicating a layer is really important just in case you make a mistake, you can easy go back to the original picture.
3. Click on the DODGE TOOL. In your Range, select MIDTONE; and, in the Exposure, choose around 30-40%. And ZOOM IN the picture around 100%.
4. Click on the areas of the teeth you want to whiten. Continue on clicking until you have achieve the whiteness you desire. Tip: Make sure your brush is not too big for the teeth.
Tip: You don't want to overdo the whiteness of the teeth as your photo would look unnatural.
If you have any questions regarding this tutorial or have any suggestions for the next tutorials, please email me. Your comments/feedbacks will be greatly appreciated.