title: "ValueError: bad transparency mask" when pasting one image onto another with Python Imaging Library?
body: <p>I'm trying to paste an image onto a backgorund with Python Imaging Library like this:</p>
  
  <pre><code>card = Image.new("RGB", (220, 220), (255, 255, 255))
  img = Image.open("/Users/paulvorobyev/test.png")
  
  ...
  
  x, y = img.size
  card.paste(img, (0, 0, x, y), img)
  
  card.save("test.png")
  </code></pre>
  
  <p>When I run this code, I get:   </p>
  
  <pre><code> "ValueError: bad transparency mask"
  </code></pre>
  
  <p>What did I do wrong?</p>