WP: Featured image as banner with default image if no featured image is selected

featured_image.php

<?php

// If featured image is set, use it
if ( has_post_thumbnail() ) {
  the_post_thumbnail('full');
// If no featured image, use specified image (change ID '1' to desired image)
} else {
  echo wp_get_attachment_image(1, 'full');
}