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('bg_img'); 
  if( w > h ) { 
    elem.width = w; 
    elem.height = w; 
  } else { 
    elem.height = h; 
    elem.width = h; 
  } 
}

$(document).ready(function(){
  resizeBg();
  
  $(window).resize(function(){
    resizeBg();
  });
  
});