Thanks for the re-post. I know some people have done the top half as an image, but you will have to do some custom changes. This is the part in the header.php that does the top half:
<div id="page-top-bg" style="border-color: <?php echo get_theme_mod( 'page_top_border', '#595A67' ); ?>; border-bottom-color: <?php echo get_theme_mod( 'tophalf_botline', '#BCBCBC' ); ?>; background-color:<?php echo get_theme_mod( 'tophalf_bg', '#ffffff' ); ?>;"></div>
More specifically, this is the part which is tied in with the theme option settings to allow a background colour (just no image)
background-color:<?php echo get_theme_mod( 'tophalf_bg', '#ffffff' ); ?>;
You would need to take that out or comment out the code and create a custom CSS to load your background image to the "page-top-bg" container.
I'm also hoping you will be using a child theme (which is included in the theme in a folder called "child-theme"), then copy the header.php over to that once you activated the theme. This allows you to keep the theme updated without losing your changes.