imgNum=1;
function start(){
   if(imgNum==1){
       document.ad.src="top_img/2.jpg";
       imgNum=2;
   }else if(imgNum==2){
       document.ad.src="top_img/3.jpg";
       imgNum=3;
   }else if(imgNum==3){
       document.ad.src="top_img/1.jpg";
       imgNum=1;
   }
   setTimeout("start()",5000);
}