Ok, go to your admincp > styles and templates > style manager > style options > common templates > header, and find this code:
HTML Code:
<td align="$stylevar[right]">$headbanner
AFTER that, add this code:
HTML Code:
<center>
<!-- Banner Rotator by Redline -->
<SCRIPT LANGUAGE="Javascript"><!--
function banner() {
};
banner = new banner();
number = 0;
// bannerArray
banner[number++] = "<a href='redirect_url' target='_blank'><img src='banner_url' border='0'></a>"
banner[number++] = "<a href='redirect_url' target='_blank'><img src='banner_url' border='0'></a>"
banner[number++] = "<a href='redirect_url' target='_blank'><img src='banner_url' border='0'></a>"
banner[number++] = "<a href='redirect_url' target='_blank'><img src='banner_url' border='0'></a>"
banner[number++] = "<a href='redirect_url' target='_blank'><img src='banner_url' border='0'></a>"
increment = Math.floor(Math.random() * number);
document.write(banner[increment]);
//--></SCRIPT>
<!-- End Banner Rotator by Redline -->
</center>