Disable WordPress media attachment page by redirecting to the parent post. Original idea from: http://www.wpbeginner.com/wp-tutorials/how-to-disable-image-attachment-pages-in-wordpress/

image.php

<?php
/**
 * Disables the media attachment page by redirecting the user elsewhere.
 * Save as 'image.php' in theme.
 * Inspired by http://www.wpbeginner.com/wp-tutorials/how-to-disable-image-attachment-pages-in-wordpress/
 */

if ($post->post_parent) {
	wp_redirect(get_permalink($post->post_parent), 301);
} else {
	wp_redirect(site_url(), 301);
}

die();