function chgImg(imgname) {
                var imgsrc = document.images[imgname].src;
                var base = imgsrc.substring(0, imgsrc.indexOf("__"));

                if (imgsrc.indexOf("off") > 0) {
                        base += "__on";
                } else {
                        base += "__off";
                };
                base += ".gif";
                document.images[imgname].src = base;
}