Take a look at header.tpl. Check out starting about line 12 for this:
<a class="logo" href="<?php echo $homepage; ?>">
<img src="resources/<?php echo $logo; ?>" width="<?php echo $logo_width; ?>" height="<?php echo $logo_height; ?>" title="<?php echo $store; ?>" alt="<?php echo $store; ?>"/>
</a>
<?php } else if (!empty($logo)) { ?>
<a class="logo" href="<?php echo $homepage; ?>"><?php echo $logo; ?></a>
<?php } ?>
I think you can hard code the URL you want there. I would suggest installing dev_tools and clone your template and then make the changes to your cloned theme.
If I missed the mark here, someone else can correct me, but it looks like that is where you need to make the change. $homepage calls the URL in settings.