title: Saving in PNG (using PIL library) after taking ImageChops.difference of two PNG files is producing transparent (or white) PNG image file
body: <p>Here in this code I am taking difference of two same resolution PNG images ,then saving the difference,. Saving in JPEG works fine but in PNG, it produces a total tranparent PNG image file. look at the comments in the last two lines</p>
  
  <pre><code>import Image
  import ImageChops
  js_black_im = Image.open("/js_black.png")
  js_white_im = Image.open("/fb_white.png")
  diff_im = ImageChops.difference(js_black_im, js_white_im)
  diff_im.save("/js_onlytext.jpeg", "JPEG") #this works as expected
  diff_im.save("/js_onlytext.png", "PNG") #this produces a total tranparent PNG image file![js_black.png][1]![fb_black.png][2]
  </code></pre>