function preload() {
  menu01=new Image();
  menu01.src="images/menu1_over.gif";
  menu02=new Image();
  menu02.src="images/menu2_over.gif";
  menu03=new Image();
  menu03.src="images/menu3_over.gif";
  menu04=new Image();
  menu04.src="images/menu4_over.gif";
 
}

function mouseover(imgName) {
   document [imgName].src ="images/" + imgName +"_over.gif";
}

function mouseout(imgName) {
   document [imgName].src ="images/spacer.gif";
}