function setbackground()
{
   var width = 0;
   if (window.innerWidth)
      width = window.innerWidth;
   if (document.documentElement.clientWidth && (width == 0 || document.documentElement.clientWidth < width))
      width = document.documentElement.clientWidth;
   if (document.body.clientWidth && (width == 0 || document.body.clientWidth < width))
      width = document.body.clientWidth;

   if (!width)
      return;
   if (width <= 800)
   {
      document.body.setAttribute("background", "img/800back.jpg");
      return;
   }
      if (width <= 1024)
   {
      document.body.setAttribute("background", "img/1024back.jpg");
      return;
   }
      if (width <= 1280)
   {
      document.body.setAttribute("background", "img/1280back.jpg");
      return;
   }
        if (width <= 1280)
   {
      document.body.setAttribute("background", "img/1280back.jpg");
      return;
   }
   if (width <= 1440)
   {
      document.body.setAttribute("background", "img/1440back.jpg");
      return;
   }
      document.body.setAttribute("background", "img/1920back.jpg");
   return;
}