title: PIL Loading a Transparent PNG as RGB?
body: <p>I'm trying to use PIL to composite some aerial imagery and running into some trouble. I use PIL to load <a href="https://gis.apfo.usda.gov/arcgis/rest/services/NAIP/Tennessee_2016_60cm/ImageServer/exportImage?bbox=-87.1875,34.3071438563,-84.375,36.5978891331&amp;bboxSR=4326&amp;size=256,256&amp;imageSR=102113&amp;transparent=true&amp;format=png&amp;f=image" rel="nofollow noreferrer">this image</a> with this little bit of code:</p>
  
  <pre><code>composite = Image.new('RGBA', (256, 256))
  url = 'http://...'
  resp = requests.get(url)
  content = StringIO(resp.content)
  image = Image.open(content)
  composite.paste(image, (0, 0), image)
  </code></pre>
  
  <p>When I make the call to <code>composite.paste()</code>, PIL gives me the error "ValueError: bad transparency mask". When I print <code>image.mode</code>, sure enough it's simply <code>RGB</code> instead of the expected <code>RGBA</code> (which <code>paste()</code> requires).</p>
  
  <p>Where does the alpha channel on my downloaded PNG go?</p>