It’s easy to whiten teeth in a picture. Choose your magic wand tool and make sure “add to a selection” is chosen in the options bar. Click on the teeth until most of the teeth are selected as shown below. It doesn’t have to be perfect.
If you have the full version of Photoshop choose Image>Adjustments>Hue/Saturation. If you have Elements choose Enhance>Adjust Color>Hue/Saturation.
In the drop-down menu choose Yellows then slide the Saturation slider to the left and possibly move the lightness slider to the right. Don’t make the teeth too white or it will look fake.