in your index.php, between the <div id="content">
and <div class="entry">
couldn’t you insert it?
<div id="myBanner">
<a href="https://bannerlink.com"><img src="<?php bloginfo('template_url'); ?>/images/banner.png" width="450" height="50" alt="my banner" /></a>
</div>
(assumes you want the banner to be a link)
then you would be able to style #myBanner in your css to give your banner some margin/padding, etc
or you could simple drop in the same location
<div id="myBanner"></div>
and style it in css:
#myBanner {
background: transparent url('images/banner.png') no-repeat;
}
you’d have to add padding/margin most likely, and this one wouldn’t be a link, but it’s just suggestions to get started.
(Also, both these examples assume you have the file in an images folder in your theme….if not, adjust the links accordingly)