if (document.images) {
  image1on = new Image();
  image1on.src = "images/mainpage/ch1_on.gif";

  image2on = new Image();
  image2on.src = "images/mainpage/ch2_on.gif";

  image3on = new Image();
  image3on.src = "images/mainpage/ch3_on.gif";

  image4on = new Image();
  image4on.src = "images/mainpage/ch4_on.gif";

  
  

    image1off = new Image();
  image1off.src = "images/mainpage/ch1.gif";

  image2off = new Image();
  image2off.src = "images/mainpage/ch2.gif";

  image3off = new Image();
  image3off.src = "images/mainpage/ch3.gif";

   image4off = new Image();
  image4off.src = "images/mainpage/ch4.gif";
  

  }

function changeImages() {
  if (document.images) {
    for (var i=0; i<changeImages.arguments.length; i+=2) {
      document[changeImages.arguments[i]].src = eval(changeImages.arguments[i+1] + ".src");
    }
  }
}