Comment on WordPress Header Images by SEO Dave.
It’s positioning to the browser window not the main wrapper div. Should be easy to fix.
I assume you’ve added the code to the bottom of the footer.php file, you just need to move it up a little to get it within the wrapper div.
In the footer.php file find the last closing div (</div>) around 14 lines from the bottom and add it just before it.
That should result in the code being treated the same way as the searchform code and the top and left positioning will be to the wrapper not the browser window.