Audible Ink encodes imagery into sound. Take any image from your photo library and play it over your iPhone speakers or iPod headset.
This tool is similar to those that professional musicians, like Aphex Twin, use to embed imagery into their music. The vertical axis maps to frequency, and pixel brightness maps to volume, so bright pixels near the bottom will produce lower tones, and those near the top will be near ultrasound.
Many images will sound roughly like static, though with a spectrogram analyzer, like Spectrogram for the iPhone, you'll be able to clearly see your original picture. To produce clear tones, use images with thin, white lines.
This app also allows you to perform image processing algorithms on your photos such as edge detection and contrast stretching to improve the audio output and picture quality in a spectrogram.