Add a new div around the image, set the width of the div to be the same as the image, and set the margin to 0 top and auto left:
<div style="margin: 0 auto; width:468px;"><img height="60" border="0" width="468" alt="Banner" src="http://banner.1and1.com/xml/banner?size=2%26%number=1"/></div>