Change header image

I am using the Graphene theme for https://www.vibetracks.com and trying to change the header image. I looked in other posts about this topic, but the header code doesn’t match and I can’t figure out how to change the image.

This is the code I see:

<div id=”header” style=”background-image:url(<?php echo $header_img; ?>);”>
<h1 <?php echo $style; ?> class=”header_title”> href=”<?php echo home_url(); ?>”><?php bloginfo(‘name’); ?></h1>

Please help.