function _isLoadedImage(node) {
    try {
      return node.complete && !node.mozMatchesSelector(":-moz-broken");
    } catch(e) {
      if (node.complete && node.readyState === "complete") {
        return true;
      }
    }
  }
Check whether the given node is a proper loaded image
FIXME: Returns undefined when unknown (Chrome, Safari)