"When you see it, you'll shit bricks." is a popular caption for many demotivational posters. The phrase expresses what one will feel after they discover the hidden image or message in the poster. The images are often referred to as "Mind Games" and "Mindfucks" also.
The poster is so commonly referenced that a site was put up just for this macro. The site was registered on April 9, 2008, but google trends shows that it became popular previous to the site's existence.