title: OS/X iconutil support for jpeg
body: <p>I am trying to use iconutil to generate icns file from iconset directory. If the icon set directory contains the required file in png format, it works. If it contains the required files in jpg format, I get </p>
  
  <pre><code>/private/tmp/MyIcon.iconset:warning: No image found for point size: 16 at scale: 1.
  /private/tmp/MyIcon.iconset:warning: No image found for point size: 16 at scale: 2.
  /private/tmp/MyIcon.iconset:warning: No image found for point size: 32 at scale: 1.
  /private/tmp/MyIcon.iconset:warning: No image found for point size: 32 at scale: 2.
  /private/tmp/MyIcon.iconset:warning: No image found for point size: 128 at scale: 1.
  /private/tmp/MyIcon.iconset:warning: No image found for point size: 128 at scale: 2.
  /private/tmp/MyIcon.iconset:warning: No image found for point size: 256 at scale: 1.
  /private/tmp/MyIcon.iconset:warning: No image found for point size: 256 at scale: 2.
  /private/tmp/MyIcon.iconset:warning: No image found for point size: 512 at scale: 1.
  /private/tmp/MyIcon.iconset:warning: No image found for point size: 512 at scale: 2.
  /private/tmp/MyIcon.iconset:error: Invalid iconset, no resources found in iconset.
  MyIcon.iconset:error: Failed to generate ICNS.
  </code></pre>
  
  <p>Yet, the directory has these images</p>
  
  <pre><code>   4190 21 Aug 16:31 icon_128x128.jpg
     4958 21 Aug 16:31 icon_128x128@2x.jpg
     3938 21 Aug 16:31 icon_16x16.jpg
     3950 21 Aug 16:31 icon_16x16@2x.jpg
     4958 21 Aug 16:31 icon_256x256.jpg
     8030 21 Aug 16:31 icon_256x256@2x.jpg
     3950 21 Aug 16:31 icon_32x32.jpg
     3998 21 Aug 16:31 icon_32x32@2x.jpg
     8030 21 Aug 16:31 icon_512x512.jpg
    32606 21 Aug 16:31 icon_512x512@2x.jpg
  </code></pre>