The theme that you use is not using tables to create the columns, but div containers, using CSS as a "glue" to create the three-column layout. What may appear to you as a fourth columns (the space to the right of you site's content) is actually the right margin. Therefore, you can not simply add a table somewhere, but you need to look into the CSS that defines the three-column layout and add a fourth column, using exactly the same technique that the theme designer (Gizmo afaik) used to create the orginal columns. A fourth column is not a wise idea though: instead, I suggest replacing the links and galleryroll section and add you banner there instead. In fact, the links and galleryroll section is just meant as sample content that the theme designer displays to give you an idea what you could do with that space. In your template.html, find <div id="sidebar">
<h4>menu</h4>
<div class="navigate">
<ul>
{ADMIN_MENU}
{SUB_MENU}
</ul>
</div>
<h4>links</h4>
<div class="navigate">
<ul>
<li><a href="http://forum.coppermine-gallery.net/" title="Coppermine Support Forum">Coppermine Support Forum</a></li>
<li><a href="http://www.mozilla.com/firefox/" title="Get Firefox">Get Firefox</a></li>
<li><a href="http://chrispederick.com/work/webdeveloper/" title="Web Developer Extension">Web Developer Extension</a></li>
</ul>
</div>
<h4>galleryroll</h4>
<div class="navigate">
<ul>
<li><a href="http://www.billygbullock.com/" title="Gizmo">Gizmo</a></li>
<li><a href="http://gaugau.de/" title="GauGau">GauGau</a></li>
<li><a href="http://stramm.st.funpic.org/" title="ModPack!">Stramm</a></li>
<li><a href="http://www.ic-gallery.com/" title="Nibbler">Nibbler</a></li>
</ul>
</div>
</div> <!-- end sidebar -->
and replace with <div id="sidebar">
<!-- your banner here -->
</div> <!-- end sidebar -->