I used this code a while back on my invisionfree board. Its pretty simple but it does the job. I didn't make it though so don't give me the credit
<script language="JavaScript">
<!--Made By Lee20 Of InvisionFree-->
<!-- Begin
var ad_cnt1 = 4;
var now1 = new Date()
var sec1 = now1.getSeconds()
var ad1 = sec1 % ad_cnt1;
ad1 +=1;
if (ad1==1) {
url1="YOUR-DOMAIN-HERE";
banner1="URL_OF_BANNER";
width1="468";
height1="60";
alt1="ALT TEXT HERE";
}
if (ad1==2) {
url1="YOUR-DOMAIN-HERE";
banner1="URL_OF_BANNER";
width1="468";
height1="60";
alt1="ALT TEXT HERE";
}
if (ad1==3) {
url1="YOUR-DOMAIN-HERE";
banner1="URL_OF_BANNER";
width1="468";
height1="60";
alt1="ALT TEXT HERE";
}
if (ad1==4) {
url1="YOUR-DOMAIN-HERE";
banner1="URL_OF_BANNER";
width1="468";
height1="60";
alt1="ALT TEXT HERE";
}
document.write('<center><a href="' + url1 + '" target="_blank">');
document.write('<img src="' + banner1 + '" width=' + width1 + ' height=' + height1 + ' border=0 alt="' + alt1 + '"></a>');
document.write('</center>');
// End -->
</SCRIPT>