Up close you'll probably see Einstein and from across the room, you'll see Marilyn Monroe, but short-sighted people see Marilyn from close up too. Original image 2007 Dr. Oliva in New Scientist. Linked today from Ace of Spades to Life Hacker to an MIT page to a New Scientist preview and from there with a $1.94 pay link you may connect to the complete New Scientist story. Life is full of chains of "Thank You's".
A scientific explanation is available at the first link from Ace:
So how does it work? The answer is that this image is a mix of low frequency elements from a picture of Monroe and high frequency details from a picture of Einstein. Normally our brain pays more attention to higher frequency components of an image so if you have normal vision, you'll tend to see Einstein's face. But if you're near-sighted, the high frequency details are blurred and so you'll just see Monroe.
What does that mean? High frequency elements are edges that define something and low frequency elements are smooth variations like colour and texture. Debugmode.com