Link Featured Images to Their Post

<?php
if ( ! function_exists( 'scseed_link_post_thumbnail' ) ) {
    function scseed_link_post_thumbnail( $html, $post_id, $post_thumbnail_id, $size, $attr ) {
        // Don't wrap on singular views — image is already in context
        if ( is_singular() || empty( $html ) ) {
            return $html;
        }
        $permalink = get_permalink( $post_id );
        $title     = esc_attr( get_the_title( $post_id ) );
        return '<a href="' . esc_url( $permalink ) . '" title="' . $title . '" tabindex="-1" aria-hidden="true">'
               . $html . '</a>';
    }
    add_filter( 'post_thumbnail_html', 'scseed_link_post_thumbnail', 10, 5 );
}