This is something called a stereogram.  It's also known by the name "holusion art."  You may have seen posters, calendars, or books of this kind of art.  It works by splitting a three dimensional image into two separate images, one for the right eye and one for the left.  Once the images are separated they are printed onto one flat surface.  In order to view this kind of image you must focus your eyes on a point behind the printed surface so that your eyes combine the two images into a three dimensional image once again. 

Try this:  Hold up your index fingers 3-24 inches in front of your face so that they are pointing toward one another.  Focus your eyes on a distant point and move your fingers toward one another until you see a third object floating between the two fingers.  This is the exact same thing that happens when you look at a stereogram correctly.  Two images combine to form one image. 

if you want to learn more or if you want to get the software to create these, you can do so by searching the internet for "holusion," "Holusion art," and "stereogram."

The "key" to this image can be found below.

This is what the above stereogram looks like once it is viewed properly.  Of course the image above is 3-D and not solid black and white.