Aqui te paso un codigo para colocar banner aleatoriamente
espero que te sirva saludos
<SCRIPT LANGUAGE="JavaScript">
<!-- 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="http://www.tupagina.com/pagina.htm";
alt="esta es la pagina ....";
banner="banner/bannerbr.gif";
width="392";
height="60";
}
if (ad==2) {
txt="";
url="www.tupagina.com/pagina2.htm";
alt="pagina dos";
banner="banner/juegos.gif";
width="392";
height="60";
}
document.write('<right>');
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('</right>');
// End -->
</SCRIPT>
