js自动切换图片

DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
html xmlns="http://www.w3.org/1999/xhtml">
 head>
  title> new document title>
  meta http-equiv="Content-Type" content="text/html;charset=UTF-8" />
  meta name="author" content="" />
  meta name="keywords" content="" />
  meta name="description" content="" />
  link rel="stylesheet" type="text/css" href="" />
  style type="text/css">style>
  script type="text/javascript">
    window.onload=init;
    var n=1; //图片标记数
    var dingshi; //让图片动的定时器
    function init(){
        dingshi=window.setInterval("tupian()",1000);
    }
    //更换图片
    function tupian(){
        var obj=document.getElementById("img1");
        n++;
        if(n>=4){
            n=1;
        }
        obj.src="yaj_"+n+".jpg";
    }
    //鼠标放上图片停止
    function stoptupian(){
        window.clearInterval(dingshi);
    }
    //鼠标离开图片动起来
    function dongqilai(){
        dingshi=window.setInterval("tupian()",1000);
    }
  script>
 head>
 body>
  img src="yaj_1.jpg" onmouseover="stoptupian()"  onmouseout="dongqilai()" id="img1"> 
 body>
html>