var aUrl = new Array(4);
var aImg = new Array(4);
var Rnd = getRandom(aUrl.length);
var lnk;

aUrl[0] = "/site/default.asp";
aImg[0] = "/images/topo/img_01.jpg";

aUrl[1] = "/site/default.asp";
aImg[1] = "/images/topo/img_02.jpg";

aUrl[2] = "/site/default.asp";
aImg[2] = "/images/topo/img_03.jpg";

aUrl[3] = "/site/default.asp";
aImg[3] = "/images/topo/img_04.jpg";

lnk = (aUrl[Rnd-1].length>0?"<a href=\""+aUrl[Rnd-1]+"\">$img$</a>":"$img$");
lnk = lnk.replace("$img$","<img src=\"" + aImg[Rnd - 1] + "\" border=\"0\" alt=\"\">");

window.document.write(lnk);

function getRandom(maxNum)
{
	if (Math.random && Math.round)
	{
		var ranNum= Math.round(Math.random()*(maxNum-1));
		ranNum+= 1;
		return ranNum;
	}
	else
	{
		today= new Date();
		hours= today.getHours();
		mins=   today.getMinutes();
		secn=  today.getSeconds();
		if (hours == 19)
			hours=18;
		var ranNum= (((hours+1)*(mins+1)*secn)%maxNum)+1;
		return ranNum; 
	}
}