con este script te funciona
Código PHP:
<script LANGUAGE="JavaScript">
<!-- This script and many more are available online from -->
<!-- The JavaScript Source!! http://javascriptsource.com -->
<!-- Begin
var how_many_ads = 2;
var now = new Date()
var sec = now.getSeconds()
var ad = sec % how_many_ads;
ad +=1;
if (ad==1) {
txt="";
url="link de hipervinculo";
alt="BANNER 2";
banner="url de la imagen";
width="145";
height="196";
}
if (ad==2) {
txt="";
url="link de hipervinculo";
alt="BANNER 3";
banner="url de la imagen";
width="145";
height="196";
}
document.write('<center>');
document.write('<a href=\"' + url + '\" target=\"_top\">');
document.write('<img src=\"' + banner + '\" width=')
document.write(width + ' height=' + height + ' ');
document.write('alt=\"' + alt + '\" border=0><br>');
document.write('<small>' + txt + '</small></a>');
document.write('</center>');
// End -->
</script>