Gifshuffle
From Just Solve the File Format Problem
gifshuffle conceals messages in GIF images by shuffling the colourmap, which leaves the image visibly unchanged. gifshuffle works with all GIF images, including those with transparency and animation, and in addition provides compression and encryption (using the ICE encryption algorithm) of the concealed message.
Any list of n items can be sorted n! ways, meaning that any particular ordering can represent a number in the range [0, n!-1]. This number can in turn store approximately log2(n!) bits of information. Thus, a GIF image with 256 colours can store up to 1683 bits (210 bytes) of information by shuffling the colours in its colourmap.
