if (document.images) {
   var redBall = new Image;
   redBall.src = "graphics/button_on.gif";
   var blueBall = new Image;
   blueBall.src = "graphics/button_off.gif";
}

//Rollover function.
function rollOver(imgName,newImg) {
   if (document.images) {
      document.images[imgName].src=newImg.src;
   }
}