Automatically Link Featured Images to Posts on WordPress

functions.php

<?php
**
 * Wrap the thumbnail in a link to the post.
 * Only use this if your theme doesn't already wrap thumbnails in a link.
 *
 * @param string $html The thumbnail HTML to wrap in an anchor.
 * @param int    $post_id The post ID.
 * @param int    $post_image_id The image id.
 *
 * @return string
 */
function wpcode_snippet_autolink_featured_images( $html, $post_id, $post_image_id ) {
	$html = '<a href="' . get_permalink( $post_id ) . '" title="' . esc_attr( get_the_title( $post_id ) ) . '">' . $html . '</a>';

	return $html;
}

add_filter( 'post_thumbnail_html', 'wpcode_snippet_autolink_featured_images', 20, 3 );