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Link all Post Thumbnails

/**
 * Link all post thumbnails to the post permalink.
 *
 * @param string $html          Post thumbnail HTML.
 * @param int    $post_id       Post ID.
 * @param int    $post_image_id Post image ID.
 * @return string Filtered post image HTML.
 */
function wpdocs_post_image_html( $html, $post_id, $post_image_id ) {
    $html = '<a href="' . get_permalink( $post_id ) . '" alt="' . esc_attr( get_the_title( $post_id ) ) . '">' . $html . '</a>';
    return $html;
}
add_filter( 'post_thumbnail_html', 'wpdocs_post_image_html', 10, 3 );

Quelle: https://developer.wordpress.org/reference/functions/the_post_thumbnail/