A lot of SMF themes have the ability to have a banner above the forum. Once this has been uploaded to the proper directory an entry is entered in the Admin section to make it link back to the primary web site. The coding I used is below and has worked well.
<div align="center">
<table border="1" cellspacing="0" width="798" style="border-collapse: collapse"
height="80" bordercolor="#FFFFFF" id="table1" cellpadding="0">
<tr>
<td><a target="_blank" href="http://www.domainname.com/"><img border="0"
src="http://www.domainname.com/smf/Themes/helios_multi11final/images/forum_header.jpg"
width="798" height="80"></td>
</tr>
</table>
</div>