Electronic Musician has an interesting article penned by Dennis Miller that looks at 8 tools for turning photographs into audio and manipulating audio visually:

Eerybody knows that a picture is worth a thousand words. But did you know that a picture can also buy you the frequencies of a thousand oscillators?

Using any of several modern music applications, you can convert the data that forms a picture on the computer screen into information that will generate an audio file. In most cases, the program uses the vertical position of each pixel of the image to control frequency and one or more color (RGB) values for parameters such as amplitude and stereo position.

Applications covered include:

1. Adobe Audition 3,
2. Thomas Baudel’s HighC 2.2
3. Camel Audio’s Cameleon 5000 1.5
4. Rasmus Ekman’s CoagulaLight 1.66
5. Nicolas Fournel’s AudioPaint 2.1
6. Image Line FL Studio 8
7. U&I Software MetaSynth 4
8. VirSyn Poseidon 1.4

It would have been nice if the online edition of this article had included audio or video demos of these apps, which would have made it more clear why these can be useful tools.