There is no config option in the forum's admin section to add another picture. To do this, you will have to edit inc_top.asp manually.
You can, for example, add an image tag right after the one which you have inserted using the Main Forum Config option. (Your additional image should not be too big otherwise it will destroy the forum layout).
You code would then look something like this (around line 152):
<table align="center" border="0" cellPadding="0" cellSpacing="0" width="100%">
<tr>
<td valign="top" width="50%"><a href="default.asp"><img alt="<% =strForumTitle %>"
border="0" src="<%=strTitleImage%>"></a>
<img src="yourimage.gif">
</td>
But be careful with editing the file. Use a text editor like notepad (not frontpage or the like!), don't mess up with the asp code and don't forget to backup your file before changing anything...
Good luck,
Barbara