Check this out! I encoded an image into an audio file using enscribe.
(Here's the .wav file if you want to listen to it, mostly sounds like hissing and static; it's only like 8 seconds long)

The image on the left is the original cool image of my buddy Kevin W. Jolly I grabbed from my "PICS" folder.
The image on the right is what the audio spectrum looks like when viewing the audio file with an audio spectrum analyzer like baudline.



Here's another example using an different image.



You can also do this with audio formats like .mp3 but the quality of the image isn't as clean.

I did this on my main Linux workstation (openSUSE 10.3) using this command:
enscribe -oversample -lf=2 -hf=70 -color=yb -wav deus_ex-m.png deus_ex-m.wav

I launched baudline to look at the .wav file like this:
baudline deus_ex-m.wav

NOTE: I had to download and fuss a bit with baudline and enscribe to make them work for me. It wasn't that big a deal though, just read through the README's carefully.

There's lots more info on encoding images into audio files on the enscribe website:
http://www.coppercloudmusic.com/enscribe/