title: What options for convert (ImageMagick or GraphicsMagick) produce the smallest (filesize) PNG?
body: <p>ImageMagick creates some pretty large PNGs.  GraphicsMagick is a lot better, but I'm still looking for the best options to use with convert to obtain the smallest filesize png.</p>
  
  <p>I have here a <a href="http://c0006847.cdn2.cloudfiles.rackspacecloud.com/big_yet_small.png" rel="noreferrer">large png with a small filesize</a>, and passing this through IM convert I have been unable to reach that filesize, let alone get it smaller.  With GM convert I can get it slightly smaller but I'm looking for improvements, generically for <strong>any image</strong> I come across.</p>
  
  <pre><code>gm convert -quality 95 a_png.png gm.png
  convert -quality 95 -depth 8 a_png.png im.png
  gm identify *
  
  a_png.png PNG 2560x2048+0+0 PseudoClass 256c 8-bit 60.1K 0.000u 0:01
  gm.png[1] PNG 2560x2048+0+0 PseudoClass 256c 8-bit 60.0K 0.000u 0:01
  im.png[2] PNG 2560x2048+0+0 DirectClass 8-bit 130.2K 0.000u 0:01 
  </code></pre>
  
  <p><strong>What options for convert produce the smallest PNG filesize?</strong></p>
  
  <p>(Yes, I'm familiar with OptiPNG, PNGOUT and Pngcrush.  But I'm after something that will be available without question on every *nix box I happen to be on.)</p>