@Jonathan: Ok is this is not working for you then you can build child theme and create functions.php file and add the following code
<?php
/**
 * Header Image
 *
 * Uses Custom Header and Featued Images
 * @Hooked in catchevolution_headercontent
 * @since Catch Evolution 1.0
 */
function catchevolution_featured_header() { 
	global $wp_query, $post, $paged, $_wp_default_headers;
	
	// Header Image
	$header_image_path = get_header_image();
	// Check if this is a post or page, if it has a thumbnail, and if it's a big one
	if ( is_singular() && current_theme_supports( 'post-thumbnails' ) &&
			has_post_thumbnail( $post->ID ) &&
			( /* $src, $width, $height */ $image = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), 'post-thumbnail' ) ) &&
			$image[1] >= HEADER_IMAGE_WIDTH ) :
		// Houston, we have a new header image!
		echo '<div id="header-image">';
			echo get_the_post_thumbnail( $post->ID );
		echo '</div>';
	elseif ( get_header_image() ) : ?>
		<div id="header-image">
			<img src="<?php header_image(); ?>" width="<?php echo get_custom_header()->width; ?>" height="<?php echo get_custom_header()->height; ?>" alt="" />
		</div>
	<?php endif; 
	
	
} // catchevolution_featured_header