There's another way to hide the image, and that is to exploit the nonlinearity of the response curves (gamma).

I have an image I crafted a long time ago which looks something like gray noise when you open it up, but when you downscale it, you see an image of Lt Cmdr Data from Star Trek. I wonder if I can dig it up.

The technique itself was not novel when I did it, a more sophisticated version involving embedded gamma values (which you can make quite large or small) was routinely used on image boards some ten or fifteen years ago.