Adding a banner or image to the Left Navigation should be easy enough.
the file you want edit is layout.tpl in your templates/default folder (or whatever template you are using.)
Put the image in your templates/default/images folder
and add this:
Code:
<div>
<img src="{$smarty.const.URL}/templates/default/images/yourimage.jpg" alt="" />
</div>
after this:
Code:
<div class="left-box">
<ul class="sidemenu">
<li><a href="{$smarty.const.URL}/index.php">{$smarty.const.LANG_TPL_HOME}</a></li>
<li><a href="{$smarty.const.URL}/search.php">{$smarty.const.LANG_TPL_SEARCH}</a></li>
{foreach from=$templatepages item="entry"}
<li><a href="{$smarty.const.URL}/pages.php?page={$entry.pageID}">{$entry.pPageTitle}</a></li>
{/foreach}
<li><a href="{$smarty.const.URL}/contactus.php">{$smarty.const.LANG_TPL_CONTACT}</a></li>
</ul>
</div>