Your brain does a lot of work when you look at things. You don't perceive exactly what your eyes see. You will use context to see something one way or another. [1][2]
The dress photo is deceptive because of the exposure, levels, and cropping. When some people look at it, they see a lot of light behind it, and subconsciously assume that the dress is probably in shadow. A white-gold dress in shadow would look the way the image does, so their brain says the dress is white and gold.
When others look at the dress, they instead see a dress that is illuminated by light probably behind the photographer, or maybe an over-exposure in some other way. Either way the dress is washed-out, and a blue-black dress would look that way if it were washed out. So their brain says the dress is blue and black.
In reality the dress is blue-black, but the photograph is essentially an optical illusion. Within the context of the photograph only, both ways are valid. You can even switch the way you see it if you can get your brain to "reset" the assumptions it's making about the image. For those of you seeing white-gold, try looking at an upside-down version of the image. [3]
Here is an image that might do a better job at demonstrating what I'm trying to convey. [4]
The dress photo is deceptive because of the exposure, levels, and cropping. When some people look at it, they see a lot of light behind it, and subconsciously assume that the dress is probably in shadow. A white-gold dress in shadow would look the way the image does, so their brain says the dress is white and gold.
When others look at the dress, they instead see a dress that is illuminated by light probably behind the photographer, or maybe an over-exposure in some other way. Either way the dress is washed-out, and a blue-black dress would look that way if it were washed out. So their brain says the dress is blue and black.
In reality the dress is blue-black, but the photograph is essentially an optical illusion. Within the context of the photograph only, both ways are valid. You can even switch the way you see it if you can get your brain to "reset" the assumptions it's making about the image. For those of you seeing white-gold, try looking at an upside-down version of the image. [3]
Here is an image that might do a better job at demonstrating what I'm trying to convey. [4]
[1] http://lesswrong.com/lw/290/blue_and_yellowtinted_choices/
[2] http://en.wikipedia.org/wiki/Checker_shadow_illusion
[3] http://s4.photobucket.com/user/ometauru/media/YNOPKWp_zpstcv...
[4] http://i.imgur.com/9N4KNLn.jpg