title: Error in images2gif.py with GlobalPalette
body: <p>Getting an error when trying to convert sequence of jpgs to gifs. Can't seem to figure out how to add a palette, or if that's the actual problem. Was able to get gifs to load using the numpy arrays in the <a href="http://visvis.googlecode.com/hg/vvmovie/images2gif.py">images2gif.py</a> main.</p>
  
  <pre><code>import PIL
  from PIL import Image
  import StringIO
  import images2gif
  
  images = []
  for frame in animation1.frames:
      img_data = s3manager.get_file_as_string(frame.s3_filename)
      image = Image.open(StringIO.StringIO(img_data))
      images.append(image)
  
  images2gif.writeGif('lala3.gif', images, duration=0.5, dither=0)  
  </code></pre>
  
  <p>With this I get the following error:</p>
  
  <pre><code>"images2gif.py", line 436, in writeGifToFile
    fp.write(globalPalette)
  TypeError: must be string or buffer, not None
  </code></pre>
  
  <p>Not sure how to specify a palette for these jpgs. documentation unclear to me, and not even sure if that's the issue. help?</p>