function rollUp(imageName)
    {

      if (document.images)
        {
          document[imageName].src=eval(imageName+'_up.src');
        }
    }

  //Image name that you want to roll down 
  function rollDown(imageName)
    {

      if (document.images)
        {
          document[imageName].src=eval(imageName+'_down.src');
        }
    }