title: iconutil error: "Unsupported image format"
body: <p>I've been trying to use iconutil to generate .icns icons from the .png images inside the folder "folderthumb.iconset", with the following command:</p>
  
  <pre><code>iconutil -c icns folderthumb.iconset
  </code></pre>
  
  <p>Everything goes well when the source png have alpha transparency. However, when the PNGs are opaque (from sips, hasAlpha=no) iconutil returns the error: </p>
  
  <blockquote>
    <p>Unsuported image format</p>
  </blockquote>
  
  <p>My libpng is v1.6, installed with <code>brew</code>.</p>
  
  <p>Has anyone tips on how to sort this problem out?</p>