title: PIL - Add transparency around .png
body: <p>I have several .pngs that are 586 x 428.  I need to convert them into .pngs that are 512 x 512.  I want to do this without stretching the image at all.  So basically, I want to scale the width down to 512, then add an equal amount of transparency in the height both above and below the original image.</p>
  
  <p>So, the final product would be a .png that is 512 x 512, and 69px of transparency in height on top and bottom, leaving the "colored" part of the image to be 512 x 374.</p>
  
  <p>I am trying to do this with Python Imaging Library.  I am pretty new to this, so if someone can help me with this, that would be much appreciated.</p>