Open up your image in PhotoFiltre.
Using the Selection Tool on the Polygon setting, zoom in on the mouth really far in so that the image is very pixely.
Now on one side of the teeth a little near the edges of the mouth, make triangles on one tooth.
Go to Filter >> Color >> Colored Layer and fill it with White 100%.
Now on the other side an equal length from the edge as the other side, move the selection over and fill in the selection.