The documentation for the Overlay Header Image shows the example
HTML Code:
<div id="header_image_sociable">
<ul>
<li><a href="http://www.facebook.com/myid">
<img src="<?php echo site_url(); ?>/wp-content/images/facebook.jpg" alt="Facebook" /></a></li>
<li><a href="http://www.twitter.com/myid">
<img src="<?php echo site_url(); ?>/wp-content/images/twitter.jpg" alt="Twitter" /></a></li>
</ul>
</div>
since you only have one item and you can't use the php it would look like this
HTML Code:
<div id="header_image_sociable">
<a href="http://yourdomain.com">
<img src="http://yourdomain.com/wp-content/images/facebook.jpg" alt="Facebook" /></a> </div>
and then you would use the css
HTML Code:
#header_image_sociable {position: absolute; right:40px; top: 20px;}
you can change the class name to suit your needs and the positions to adjust the logo where you want it.