function buttondown( buttonname )
{
    if (document.images) {
      
      var imageOriginal = document[ buttonname ].src;
      var imageReplaced = imageOriginal.replace("-off.gif", "-on.gif");
      
      document[ buttonname ].src = imageReplaced;
    }
}
function buttonup ( buttonname )
{
   if (document.images) {
      
      var imageOriginal = document[ buttonname ].src;
      var imageReplaced = imageOriginal.replace("-on.gif", "-off.gif");
      
      document[ buttonname ].src = imageReplaced;
    }
}