function swapBg()
{
  if(document.getElementById)
  {
    var thePicture=document.getElementById("fondos");
    var picPath="backgrounds/fondo"+rnd(10)+".jpg";
    thePicture.style.background="url("+picPath+")";
  }
}
     
function rnd(n)
{
  return Math.floor(Math.random() * n) + 1;
}