图片无法显示时,js实现 暂无图片效果

<script type="text/javascript">
window.onload=swapimg;
function swapimg()   
      {
        var img=document.getElementsByTagName("img")
        for(var i=0;i<img.length;i++){
            if (img[i].readyState=="uninitialized"){
            img[i].src="images/nav_06.png";
            }
        }
     }
</script>