var timer, speed = 175;
function flash()
 {
head_html = document.getElementsByTagName('head')[0].innerHTML;
if (head_html.indexOf('href="on.png"') != -1)
document.getElementsByTagName('head')[0].innerHTML = head_html.replace('on.png', 'off.png');
else
document.getElementsByTagName('head')[0].innerHTML = head_html.replace('off.png', 'on.png');
timer = setTimeout('flash()', speed);
 }

function kill_flash() {clearTimeout(timer);}

flash();