$(document).ready(
       function ()
       {
        change_head_img();
       }
     );
function change_head_img()
{
  var randomnumber= parseInt(Math.random()*3);
  var valor="images/"+randomnumber+".jpg"
  if(valor!=$("#header_center_picture img").attr("src"))
  {
     var src_img_div='url(\''+valor+'\')';
     $("#header_center_picture").css('background-image',src_img_div);
     $("#header_center_picture img").fadeOut("slow",function(){
         $("#header_center_picture img").attr("src",valor);
      });
      $("#header_center_picture img").fadeIn("slow");
    setTimeout("change_head_img()",4000);
  }
  else
      change_head_img();
}