The alleged Russian spies recently arrested by the FBI are accused of encoding messages into otherwise innocuous pictures, marking the first confirmed use of this high-tech form of data concealment in real life, experts say...
Although computer scientists have theorized about the existence of this communication technique for over a decade, this is the first publicly acknowledged use of the technique.
Programmers do stuff like this just for fun, and you can learn how at Ben Nadel's blog.
And spies probably do, too. (Drawn in ArtRage2 with drybrush and grain filters added in Photoshop. That gave it kind of a nice watercolor look.)