var resizeBg = function() { 
  var h = self.innerHeight || document.documentElement.clientHeight || document.body.clientHeight;
  var w = self.innerWidth || document.documentElement.clientWidth || document.body.clientWidth;
  var elem = document.getElementById('backgroundImage');

  if (w > h) {
    elem.width = w;
    elem.height = w;
  } else {
    elem.height = h;
    elem.width = h;
  }
}
//--> 
$(window).resize(function(){
     resizeBg()
});