You don't need special toothpaste if you've got a photo editor! You can whiten and brighten your smile in just a few steps using layers and blending modes in the free photo editor Paint.NET
Time Required: 3 minutes
- Open the photo you wish to work with.
- Create a new blank layer above the background.(Layers > Add New Layer)
- Zoom in on the teeth in your photo.
- Select the Paintbrush tool in the tools palette.
- Set the primary color to white in the Color palette.
- Adjust your brush size in the tool options bar so that it is slightly smaller than the teeth. You can also use the square  bracket keys to adjust the brush size up and down as you paint.
- In the layers palette, double-click on the top layer to open Layer Properties.
- Set the layer Opacity to about 125, type "whiten teeth" for the layer name and click OK.
- Paint white over the teeth, being careful not to go over the lips. It's okay if you don't cover every bit of tooth area--in fact, it's better if you stay a few pixels away from the gums and lips.
- If necessary, switch to the eraser tool and clean up any areas where you may have painted over the lips or gums.
- Set the zoom level on your photo to 100%. (View > Actual Size)
- Go to Effects > Blurs > Gaussian Blur.
- Set the radius to a number between 1-10. Use your judgment; the amount of blur you need will depend on the resolution of the photo. You want the white paint to gradually fade out so there is no obvious line.
- At this point the whitening will probably look to harsh and unnatural. Go back to Layer Properties, and set the blending mode to Overlay, and drop the opacity down further, until the effect is natural looking. My image required an opacity setting around 80.
- Click the checkbox next to the "whiten teeth" layer in the layers palette to compare the before and after results.
- Don't go too extreme with the whitening--it will look obviously faked.
- Flatten your layers and save the image under a new file name so you don't overwrite your original.