function addLoadEvent(func){
  var oldonload = window.onload;
	if(typeof window.onload != 'function'){
	  window.onload = func;
	} else {
	  window.onload = function(){
		  oldonload();
			func()
		}
	}
}
function swapImage(){
	if (!document.getElementsByTagName) return false;
	var main = document.getElementById('content-main');
	var list = main.getElementsByTagName('ul');
	var images = list[0].getElementsByTagName('img');
	for(var i=0;i<images.length;i++){
		var imageSrc = images[i].getAttribute("src");
		var pngIndex = imageSrc.indexOf("png");
		imageSrc = imageSrc.substring(0,pngIndex);
		imageSrc += "gif";
		images[i].setAttribute("src",imageSrc);
	}
}

addLoadEvent(swapImage);