title: ImageMagick: `convert` is modifying png24 to png8, any solution?
body: <p>I am converting some high-res iPhone images to low-res with batch processing using ImageMagick. I simply run the following:</p>
  
  <p><code>convert image@2x.png -resize '50%' image.png</code></p>
  
  <p>The problem is using <code>pngcheck</code>, it appears that the original @2x file is 32bit (where 8bits are an alpha channel) and in the down sized version, that is stripped away.</p>
  
  <p>pngcheck output (before convert):  </p>
  
  <p>OK: image@2x.png (85x81, 32-bit RGB+alpha, non-interlaced, 94.6%). </p>
  
  <p>pngcheck output (after convert):</p>
  
  <p>OK: image.png (43x41, 8-bit palette+trns, non-interlaced, 4.8%).</p>