It wasn't simply stenography that Valve used for this puzzle. Saying as much actually takes away from the awesomeness of what Valve has done here.
Amateur radio (sometimes called ham or short wave) operators have various ways of communicating with one another. One obvious method of course is voice. Another is morse code, which in fact is present in the updated Portal radios. Check the Steam forums for more info on what this was decoded into.
But another method of communication is called SSTV, for Slow-Scan Television. It basically means sending images over the air waves using basically the same kind of tones which you'd hear from a modem over a phone line. Those with even novice experience at amateur radio would probably recognize the distinctive sound of an SSTV transmission, which is probably what Valve was counting on.
So yes, someone at Valve had the very clever idea of including SSTV broadcast signals INSIDE of the game, via the radios you can carry around. People immediately extracted the individual audio files from Portal's data files so that they could work with it, then ran it all through software which people normally would use for demodulating SSTV broadcasts from a real radio. Each audio stream produced an image.
Since SSTV is transmitted through audible sounds, any interference/noise introduced can cause a corrupted image. You'll notice in all the SSTV images retrieved so far that they contain static. This was apparently introduced by Valve on purpose. Whether this was simply supposed to emulate the realism of SSTV, or whether it's actually more information encoded into the stream as well, nobody seems to know as of yet. My bet is just on the realism, but who knows.
So, between using the morse code, SSTV, a real functioning BBS, ASCII artwork, etc, Valve has gone all out this time on this puzzle. They get massive geek love.