The Stego Gallery is a collection of steganography examples. The images in this gallery have been modified to include hidden messages.
Steganography is the practice of hiding a message or data within another medium. The Wikipedia steganography page is a good primer on history and techniques.
This gallery was started as fun project for other steganography enthusiasts. The images are created using a simple tool that has not been released in the wild.
The messages are hidden with varying degrees of difficulty. Some of the images even have clues within the picture to aid in finding the message. Some of the messages are plaintext, while others are encrypted.
No, it's not possible to see the hidden message with the naked eye. You might see a hint though.
The steganography software used here was started as a personal project. New functions are continually being added to the tool. There are no intentions to release the software to the public.