Looks like you can add menus or any other HTML markup to the simpleviewer by editing your index.htm file. They've included a template with a header and footer at
SimpleViewer
You can click on the link at the top to download the template.
Once downloaded, just add the link to your website between the <div=header> or <div=footer> tags depending upon if you want it at the top or the bottom.
You can also edit the caption field of your gallery.xml file, though this would have to be done on a pic by pic basis.
<image>
<filename>wide.jpg</filename>
<caption><![CDATA[Captions can be HTML formatted. Supported tags are <b>bold</b>, <u>underline</u>, <i>italics</i>, <u><a href="http://www.google.com" target="_blank">hyperlinks</a></u>, linebreaks<br> and <font color="#ffff00" size="30">font tags</font>.]]></caption>
</image>
Add your HTML link between the <caption> tag, for greater control on where the user is directed to on your site based upon the picture that was displayed.