title: no decode delegate for this image format `PNG'
body: <p>I'm writing a django/python application and I need to convert PDF files uploaded by the user to JPEGs (files are scans)</p>
  
  <p>I'm trying to use wand, with an ImageMagick backend (on OSX) and
  I keep getting the following error:</p>
  
  <pre><code>MissingDelegateError at /user_docs/upload/certificate/
  no decode delegate for this image format `PNG' @ error/constitute.c/ReadImage/501
  </code></pre>
  
  <p>My current test code is as simple as:</p>
  
  <pre><code>with wandImage(filename='/Users/username/Pictures/telechargement.pdf') as img:
      img.format = 'jpeg'
      img.save(filename='/Users/username/Pictures/bzzz.jpeg')
  </code></pre>
  
  <p>Some information:</p>
  
  <ol>
  <li>The file exists (I verify with <code>os.path.exists</code>)</li>
  <li>I can convert the file from PDF to JPG on the command line using <code>convert</code></li>
  <li>I tried to set <code>DYLD_LIBRARY_PATH</code> to include the ImageMagick lib directory</li>
  <li><code>identify -list configure | grep DELEGATES</code>  returns</li>
  </ol>
  
  <p><code>DELEGATES     bzlib fftw fontconfig freetype gs jpeg jng lcms2 lzma mpeg png tiff x11 xml zlib</code></p>
  
  <p>Anyone has an idea?</p>
  
  <p>Thanks</p>