title: Check image transparency
body: <p>I have a website where people can upload an PNG image and save it. 
  But before they can save it, i need a check if the image contains transparency. 
  Is there an way to check (i prefer JavaScript) if an image is not 24 bits?</p>
  
  <pre><code>&lt;img id="imageId" src=#" onload="checkRestriction(this,'1')" alt=""/&gt;
  
  var isPng24Bit = false;
  
  function checkRestriction(image, id) {
      if(image.colorDepth != 24) {
        PNGis24Bit = false;
      } else {
        PNGis24Bit = true; 
      }
  }
  </code></pre>