just put <img src="http://www." alt="" width="100%" />
The 100% will make the image as width as it can be in the div it's in.
Though it's an ugly solution imo. You could better just use a centered image and give the div it's in the same background color.
For example:
HTML Code:
<!-- this in your head -->
<style type="text/css">
#header{
background: #000000;
text-align: center;
}
</style><!-- ect -->
<!-- this in your body where te banner should be -->
<div id="header">
<img src="images/banner.gif" alt="website banner" />
</div>