I wrote something like that for an ARG. Basically I used GIF images with sets of 2 visually identical entries in pallet and encoded bits by picking either the

I wrote something like that for an ARG. Basically I used GIF images with sets of 2 visually identical entries in pallet and encoded bits by picking either the first or second. Not quite as sophisticated as something like varying which dct coefficients to keep in a jpeg, but it was intended to be breakable with a hex editor. The problem was that most image hosts were smart enough to detect the sub-optimal encoding and "helpfully" fix it.