Compute backgroundImage url from css class.

computedBackgroundImage.js

computedBackgroundImage: function(className) { 
  var div = document.createElement("div"); 
  div.style.display = "none"; 
  div.className = className; 
  document.body.appendChild(div); 
  
  var url;
  var style = window.getComputedStyle(div)["background-image"];
  if (style) {
    var match = style.match(/^url\((.*)\)/);
    if (match) url = match[1];
  }
  
  document.body.removeChild(div); 
  return url; 
}