title: Python Pillow v2.6.0 paletted PNG (256) How to add an Alpha channel?
body: <p>I have a numpy array that is written to an image, a RGB colormap added as a palette, and all that remains is a transparency channel (256 values) on top. I have tried converting to RGBA, LA, and other ways around it but, I cannot figure out how to add this multi-value channel on top as a palette.</p>
  
  <p>Here is an example that I have that adds a single-value channel of transparency:</p>
  
  <pre><code># data = numpy array 1624x3856
  im = Image.fromarray(data)
  im = im.convert('P')
  # cmap is a 768-valued RGB array
  im.putpalette(my_cmap)
  im.save('filename.png', transparency=0)
  </code></pre>
  
  <p>The channel I want to save is as follows:</p>
  
  <pre><code># len(alpha) = 256
  alpha = [0,255,255,255...255,255,255]
  </code></pre>
  
  <p>Any help would be greatly appreciated.</p>