It all depends on where you want to put it at the bottom. You can do this in the footer template.
Find:
Code:
<br />
<div class="smallfont" align="center">$vbphrase
Above this add:
Code:
<br />
<center>
<td align="$stylevar[center]"><a href="http://www.link_to_your_banner.com"><img src="$stylevar[imgdir_misc]/path_to_your_image.jpg" border="0" alt="$vboptions[bbtitle]" /></a></td>
</center>
Change
http://www.link_to_your_banner.com to the link people will go to when they click on the banner.
Change
path_to_your_image.jpg to your image.
This will put the image above the time at the bottom.