title: Animated GIFs are only transparent on their first frame (Python PIL)
body: <p>The following code creates a GIF from two images:</p>
  
  <pre><code># GIFs are always palette images so they would be converted later anyway
  im1 = PIL.Image.open('grin-emoji-by-twitter-rgba.png').convert('P')
  im2 = PIL.Image.open('grin-emoji-by-twitter-rgba-2.png').convert('P')
  
  im1.save('output.gif', save_all=True, append_images=[im2, im1, im2], loop=0, duration=200, transparency=255)
  </code></pre>
  
  <p>However the result is unexpectedly wrong. The first frame is fine, but subsequent frames contain a black rectangle around the updating region instead of transparency:</p>
  
  <p><a href="https://i.stack.imgur.com/Z14KH.gif" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/Z14KH.gif" alt="enter image description here"></a></p>
  
  <p>In my opinion the error is as follows: On the first image we specify the index 255 to be the index for completely transparent color. However the <code>save</code> function only seems to convert index 255 to transparency on the first frame, but skips this step on all other frames.</p>
  
  <p>Is there any way to circumvent this?</p>