title: Cannot convert canvas to image when there is another image in it
body: <p>I want to export Raphael -> PNG.  I actually had to write my own function to parse the DOM objects.</p>
  
  <p>My canvas works but the conversion using toDataUrl() doesn't always work.  I've come to see that if there is an image on the canvas so the canvas does not export, otherwise it does.</p>
  
  <p>Is anyone familiar with this?  Is there a workaround?</p>