Where is the code?
It's all CSS/HTML.... but anything you do to any other tables on your page will screw up your header/masthead/topbanner what ever your term is... like big images on the left side, etc.... will cause it to shift your center and the header will be off set, because of differnces between ie and FireFox... it will drive you crazy.....
Here is the code i use....
<style type="text/css">
body{background-position:top center;margin-top: 500;}
div.topbanner {
background-image:url();
top: 0;
left: 50%;
margin-left: -436px;
width: 872;
height: 500px;
position: absolute;
}
</style><div class="topbanner"><IMG SRC="http://yourdomain.com/image.jpg"
border="0"></a></div>
Red = the height of the banner.
Blue = negative 1/2 of the banner width.
Light Blue = the width of your banner.
Yellow = URL to image.
It's not exact you may have to tweak the alignment by increasing or decreasing the margin-left: by a few pixels to get it right.