var how_many_ads = 3; 
var now = new Date() 
var sec = now.getSeconds() 
var ad = sec % how_many_ads; 
ad +=1; 

if (ad==1) {
banner="images/banner1.jpg"; 
alt="";
width="756"; 
height="197"; 
} 

if (ad==2) { 
banner="images/banner2.jpg";
alt="";
width="756"; 
height="197";
} 

if (ad==3) { 
banner="images/banner3.jpg"; 
alt="";
width="756"; 
height="197";
} 

document.write('<center>'); 
document.write('<img src=\"' + banner + '\" width=') 
document.write(width + ' height=' + height + ' '); 
document.write('alt=\"' + alt + '\" border=0>'); 
document.write('</center>');