call a random advertisement

[Winterworks]

I have an advertisement part in my site. This is the code (added in FORUMHOME at the bottom)

<!-- Trasion Advertisements -->
<tbody>
<tr>
<td class="thead" colspan="2">
<a style="float:$stylevar[right]" href="#top" onclick="return toggle_collapse('forumhome_stats')"><img id="Advertisement" src="$stylevar[imgdir_button]/collapse_thead$vbcollapse[collapseimg_forumhome_stats].gif" alt="" border="0" /></a>
<phrase 1="$vboptions[bbtitle]">Trasion Advertisements</phrase> 
</td>
</tr>
</tbody>
<tbody id="collapseobj_forumhome_stats" style="$vbcollapse[collapseobj_forumhome_stats]">
<tr>
<td class="alt2"><img src="$stylevar[imgdir_misc]/adv.gif" alt="<phrase 1="$vboptions[bbtitle]">$vbphrase[x_statistics]</phrase>" border="0" /></td>
<td class="alt1" width="100%">
 
<p align="center">
 
<img src="http://www.trasion.com/ads/advertisementvbnt.jpg" border="0" /> 

</center>
<br>
</td>
</tr>
</tbody>
<!-- Trasion Advertisements -->

Where it says "<img src="http://www.trasion.com/ads/advertisementvbnt.jpg" border="0" />"

I don't want just 1 image. I want it so everytime you refresh, the image will randomly be selected from a folder, or something. So it changes everytime you visit the site (but it can rarely repeat).

What is the code to do this? Please add it into my code above and repost