if (document.images)   {
     image1on = new Image();
     image1on.src="images/LAKE_ON.jpg";

     image1off = new Image();
     image1off.src = "images/LAKE_OFF.jpg";

}

function turnOn(imageName)   {
     if (document.images)   {
     document[imageName].src = eval(imageName + "on.src");
     }
}

function turnOff(imageName)   {
     if (document.images)   {
     document[imageName].src = eval(imageName + "off.src");
     }
}