function randcase()
{
    var imgurl = 'cn/images/';
    var extension = '.gif';
    var rand = Math.round(Math.random()*7)+1;
    var newimg = imgurl +"ad-2-logo" + rand + extension;
	var newimgover = imgurl +"ad-2-logo" + rand + "-over" + extension;
    document.getElementById("ad2").setAttribute("src",newimg);
	document.getElementById("ad2over").setAttribute("src",newimgover);
}