Wikipedia article. Hide and extract images within other images. For example, you can recover the cat from this image.
Ruby and the chunky_png gem (gem install chunky_png). Has been tested with Ruby 2.2.3p173 and chunky_png 1.3.5. Probably works okay with other combinations, just not tested with it.
Console environment
To encode:
$ ruby steganograte.rb HOST_FILENAME SYMBIONT_FILENAME OUTPUT_FILENAME DEPTH
host filename - The image that you want to hide another image in (i.e., this one will be visible)
symbiont filename - The image that you want to hide inside of another image (i.e, this one will be concealed)
output filename - Where to put the output image after combinging
depth - The number of shades per each of the RGB channels (this should be a power of 2. E.g., if depth=4, then there would be 4^3 = 64 total colors, with log(4) = 2 of the least significant bits per channel reserved for the hidden image)
For example:
steganograte input.png secret.png mashed.png 4
To decode:
$ ruby recoverate.rb INPUT_FILENAME OUTPUT_FILENAME DEPTH
input filename - The image that you wish to extract a secret image from
output filename - Where to put the output image after extraction
depth - Same thing as above
For example, to recover the image from the above example:
recoverate mashed.png secret_recovered.png 4
Programmatically
See the Steganogrator module in steganograte.rb and recoverate.rb, particularly the steganograte and recoverate methods. There is also a dither method included as a bonus (it is used in the stenography process to maintain image quality).
Note: it might work with depth not as a power of 2, but that has not been tested.