top_on = new Image();
top_on.src = "/images/layout1/top_on_f.gif";
top_off = new Image();
top_off.src = "/images/layout1/top_f.gif";

 function ontop() {
  document.top.src = top_on.src;
}
 function offtop() {
  document.top.src = top_off.src;
}