What I did was edit the header-banner.php file. Change the banner div with the following code:
<div id="banner">
<?php if (has_post_thumbnail()): ?>
<img src="<?php echo get_the_post_thumbnail_url(); ?>" alt="<?php bloginfo('name'); ?>-Banner">
<?php elseif (of_get_option('default_banner_image')): ?>
<img src="<?php echo of_get_option('default_banner_image'); ?>" alt="<?php bloginfo('name'); ?>-Banner">
<?php else: ?>
<img src="<?php echo get_template_directory_uri().'/includes/images/banner.jpg'?>" alt="<?php bloginfo('name'); ?>-Banner">
<?php endif; ?>
</div>
This order will show an image in this order:
1. Featured image
2. Default Banner image
3. Stock image that came with the theme
