诀窍在于media_sideload_image() 和set_post_thumbnail(). media_sideload_image() 假设您可以获取图像的URL，无论它是否存在于/wp-content/ 或者其他地方（甚至其他网站）。只要您可以通过编程方式引用图像的URL，类似这样的操作就可以了。

$image = \'image.jpg\';
$media = media_sideload_image($image, $post->ID);
if(!empty($media) && !is_wp_error($media)){
    $args = array(
        \'post_type\' => \'attachment\',
        \'posts_per_page\' => 1,
        \'post_status\' => \'any\',
        \'post_parent\' => $post->ID
    );

    // reference new image to set as featured
    $attachments = get_posts($args);

    if($attachments){
        foreach($attachments as $attachment){
            set_post_thumbnail($post->ID, $attachment->ID);
            // only want one image
            break;
        }
    }
}

• Function Reference/media sideload image « WordPress Codex

• Function Reference/set post thumbnail « WordPress Codex