Here are various versions of another switching image. On the far right is psychologist E. G. Boring's famous "Wife/Mother-in-Law" image, often found reproduced in perception textbooks. When you see a young woman looking away, much like Hunt's lost profile or the sister we saw in one of Allston's paintings, you are seeing Boring's "wife image." When you can see the face of an old lady, an old hag, a witch-like image, you are seeing Boring's "mother-in-law image."
The image on the left is thought to be the original. It was an anonymous German postcard from 1888.