How to display header image at top of posts–instead of custom or featured image

I’ve searched online for hours for a solution to this and have tried tweaking things…nothing is working.

When I set a featured image in a post, the image appears at the top of the screen, replacing the default header image I created.

I *suspect* it has something to do with this code in the .functions.php section, but I don’t know how to adjust it to get the results I want:

/**
 * Display either the custom image or default header image
 *
 * @since 1.0.0
 */
function header_images() {
	$custom_image = ( is_singular() ) ? get_post_meta( get_the_ID(), 'matheson_custom_image', true ) : '';

	if ( is_singular() && ( ! empty( $custom_image ) || has_post_thumbnail() ) ) {
		if ( $custom_image )
			echo '<img src="' . esc_url( $custom_image ) . '" alt="" class="header-img" />';
		else
			the_post_thumbnail( 'full', array( 'class' => 'header-img' ) );
	} else {
		$header_image = get_header_image();
		if ( ! empty( $header_image ) ) :
			?>
			<img class="header-img" src="<?php header_image(); ?>" width="<?php echo get_custom_header()->width; ?>" height="<?php echo get_custom_header()->height; ?>" alt="" />
			<?php
		endif;
	}
}

Any help?