优先选择你手动设置的图像，如果未手动设置，自动将文章中的第一张图片设置为特色图像。

Function代码

//自动将文章中的第一张图片设置为特色图像，by minirizhi.com
function autoset_featured() {
    global $post;
    $already_has_thumb = has_post_thumbnail($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');
add_action('save_post', 'autoset_featured');
add_action('draft_to_publish', 'autoset_featured');
add_action('new_to_publish', 'autoset_featured');
add_action('pending_to_publish', 'autoset_featured');
add_action('future_to_publish', 'autoset_featured');