Except, of course, the person for whom the message is intended. That person can separate the message from the image with software recently developed by University of Maine engineering professor Rick Eason and Kyushu Institute of Technology professor Eiji Kawaguchi. While Eason and Kawaguchi's software is new, with patent pending, steganography isn't. One ancient method of concealment was to shave the head of the messenger and tattoo the message on his head. After the messenger's hair grew in, the tattoo would be undetected until his head was shaved again.
Modern examples are invisible inks and microdots.
Hiding information among the pixels is not new either. But whereas previous steganographic efforts focused on the least significant areas of an image, which were easy to manipulate without affecting the picture, Eason and Kawaguchi use the most complex, or noisiest, parts of the image where the pixels are densest, thereby making the message that much harder to decode. Just as Waldo, for example, is usually hidden in the busiest part of the picture. While the military and industrial espionage value of steganography is obvious, Eason envisions other, more benign apps. Makes it much harder to forge.