For example, is it possible to modify the header of a image file to change it into a music file, and play this file?
Another example, is it possible to change the header of an image to make it black and white only?
It's very dependent on what types of files you're dealing with. Image files can be read as raw data and fed into an audio player, but you'll usually get noise. But there's no way to turn, say, a PNG into a MP3 this way. Turning an image from color to black and white is a bit more complex, but possible under controlled conditions.