function menu(imgobj, imgid, buttonswitch)
   {
   if ( buttonswitch == "on" )
      {
      imgobj.src = "images/menuon_0" + imgid + ".gif";
      }
   else if ( buttonswitch == "off" )
      {
      imgobj.src = "images/menuoff_0" + imgid + ".gif";
      }
   else alert("Error condition in menu.");
   }