compression algorithm
I wrote this program long ago and published, but removed it later, because could be an abuse. It is possible to write a plugin for the browser, that extracts hidden images and show them in the browser in place of originals, so some visitors will be seeing art gallery and others, who downloaded plug in, something else, may be illegal. I don't mean regular pornography, it is legal. It may be used by terrorists.
The way I did it was follows: I compressed one image and redistributed it in two least significant bits of each byte of pixel data. I used my own compression algorithm. In steganography the compressed image is called payload and where it is embedded is called carrier. My payload was half of the area of carrier. And I hid even multiple images within each other like Russian dolls. The carrier can be compressed and hidden within new carrier. However each payload was half of carrier area. I tested those program that claimed to determine steganographic content. Neither of them detected payload. However, there is a way to find that out. The carrier must be lossless, I used PNG. And that raised suspicion. Why image is posted in PNG format. Steganography is not illegal, I decided to remove my program from internet myself, there was no pressure.