Download featured image

function king_add_download($text){

    global $post;

    if(($post->post_type == 'post')){

        $thumb_URL = get_the_post_thumbnail_url( get_the_ID(), 'full' );
        $last_tag = strrpos( $text, "</" );
        $text = substr_replace( $text, '<a class="download_image" href="'.$thumb_URL.'">Download this Image</a>', $last_tag, 0 );
    }

    return $text;
}

add_filter('the_content','king_add_download', 1);