html中怎么设计几个图片交替变换？

var imgUrl=new Array();
var imgLink=new Array();
var adNum=0;
imgUrl[1]="image/a.jpg";
imgUrl[2]="image/b.jpg";
imgUrl[3]="image/c.jpg";
imgUrl[4]="image/d.jpg";
imgUrl[5]="image/e.jpg";
imgUrl[6]="image/f.jpg";
imgUrl[7]="image/g.jpg";
var imgPre=new Array();
for (i=1;i<8;i++){
imgPre[i]=new Image();
imgPre[i].src=imgUrl[i];
}

function setTransition(){
if (document.all){
imgUrlrotator.filters.revealTrans.Transition=Math.floor(Math.random()*20);
imgUrlrotator.filters.revealTrans.apply();
}
}

function playTransition(){
if (document.all)
imgUrlrotator.filters.revealTrans.play()
}

function nextAd(){
if(adNum<imgUrl.length-1)adNum++ ;
else adNum=1;
setTransition();
document.images.imgUrlrotator.src=imgUrl[adNum];
playTransition();