First go to the sister project http://scratch.mit.edu/projects/25712779/ to hide a secret text of your choice in a completely unsuspicious picture on your computer! Then use this project to retrieve the hidden text! The beautiful picture of the dresser was made by @goch. Thanks to @frodewin for sharing the secrets of RGB color sensing in Scratch!
The art of hiding text in a picture is called steganography. No one will even guess that the picture contains hidden information! For info see http://en.wikipedia.org/wiki/Steganography For an explanation of this "magic trick", see inside!