Steganography (Merriam-Webster)
- the art or practice of concealing a message, image, or file within another message, image, or file
More, including etymology at the link.
UPDATE:
Recent news provides us with an excellent example.
Suspected al Qaeda operative encrypted terrorist plans in porn file (The Verge)
...[T]he cards and drive (found in Lodin's underwear) appeared to primarily contain a pornographic movie called "Kick Ass" and a file marked "Sexy Tanja." After weeks of work, however, investigators found that the video was actually a steganographic file, hiding over a hundred hidden documents believed to have come from senior al Qaeda members.
