This amazing image looks like physicist Albert Einstein. However, move a few feet away from the screen and suddenly it'll transform into Marilyn Monroe. The work of Aude Oliva and her colleagues at the Massachusetts Institute of Technology, the illusion was created in three steps.
First, the researchers obtained a photograph of Marilyn Monroe and removed the fine-grained facial features, such as any wrinkles or other blemishes.
Second, they obtained a photograph of Albert Einstein and removed the more coarse features, such as the shape of the mouth or nose.
Finally, the two images were superimposed on top of one another. Because the fine-grained features are visible close up, the image looks like Albert Einstein when you're just a few inches away from the page. However, move a few feet away and suddenly only the coarse features are visible, magically transforming the image into Marilyn Monroe.
[Emphasis mine.] Richard Wiseman (Professor of psychology, University of Hertfordshire), Ten of the greatest optical illusions
The Marylin Einstein hybrid image was created by Dr. Aude Oliva for the March 31st 2007 issue of New Scientist magazine.
Source. This is the paywall to the reference.
We present hybrid images, a technique that produces static images
with two interpretations, which change as a function of viewing
distance. Hybrid images are based on the multiscale processing of
images by the human visual system and are motivated by masking
studies in visual perception. These images can be used to create
compelling displays in which the image appears to change as the
viewing distance changes. We show that by taking into account
perceptual grouping mechanisms it is possible to build compelling
hybrid images with stable percepts at each distance. We show examples
in which hybrid images are used to create textures that become
visible only when seen up-close, to generate facial expressions
whose interpretation changes with viewing distance, and to
visualize changes over time within a single picture.
Oliva, Torralba and Schyns, Hybrid images