code: function filterImageUrls (elem, urls) {
          var child = elem.firstChild;
          while (child) {
              if (child.tagName == 'IMG') {
                  var url = child.src;
                  if (urls.indexOf(url) == -1) {
                      urls.push(url);
                  }
              }
  
              filterImageUrls(child, urls);
  
              child = child.nextSibling;
          }
      }
docs: Recursively retrieve all image urls from the images located inside a given
  HTML element
  @param {Node} elem
  @param {String[]} urls   Urls will be added here (no duplicates)