Hello,
You'd have to do this in your index.php file open it up and look for:
<div id="s5_navv" <?php if ($is_ie6 == "yes") { ?>style="height:32px;"<?php } ?>>
'
Insert:
<div id="s5_logo" style="cursor:pointer;" onclick="window.document.location.href='index.php'"></div>
Right after the s5_navv. From here you will most likely have to adjust some CSS on the s5_logo.
Regards,