var curIndex=0;
//时间间隔 单位毫秒
var timeInterval=6000;
var arr=new Array();
arr[0]="../Images/pic_main1.jpg";
arr[1]="../Images/pic_main2.jpg";
arr[2]="../Images/pic_main3.jpg";
arr[3]="../Images/pic_main4.jpg";
setInterval(changeImg,timeInterval);
function changeImg()
{
   var obj=document.getElementById("obj_MainPic");
  if (curIndex==arr.length-1) 
  {
        curIndex=0;
  }
  else
  {
     curIndex+=1;
  }
  obj.src=arr[curIndex];
}