主题实际已经设置自动搜索文章第一张图片作为缩略图了，但是有时候图片太大就导致了缩略图也很大，加载反而变慢，又懒得每次手动设置特色图像，所以这段代码可以自动把文章中上传的第一张图片设置为特色图像，缺点就是不支持外链图片

function autoset_featured_image() {
    global $post;
    if (!is_object($post)) return;
    $already_has_thumb = has_post_thumbnail($post->ID);
    if (!$already_has_thumb)  {
        $attached_image = get_children( "post_parent=$post->ID&post_type=attachment&post_mime_type=image&numberposts=1" );
        if ($attached_image) {
            foreach ($attached_image as $attachment_id => $attachment) {
                set_post_thumbnail($post->ID, $attachment_id);
            }
        }
    }
}
add_action( 'the_post', 'autoset_featured_image' );
add_action( 'save_post', 'autoset_featured_image' );
add_action( 'draft_to_publish', 'autoset_featured_image' );
add_action( 'new_to_publish', 'autoset_featured_image' );
add_action( 'pending_to_publish', 'autoset_featured_image' );
add_action( 'future_to_publish', 'autoset_featured_image' );