title: With the Python Imaging Library (PIL), how does one compose an image with an alpha channel over another image?
body: <p>I have two images, both with alpha channels.  I want to put one image over the other, resulting in a new image with an alpha channel, just as would occur if they were rendered in layers.  I would like to do this with the Python Imaging Library, but recommendations in other systems would be fantastic, even the raw math would be a boon; I could use NumPy.</p>