对于每个图像/帖子组合，请运行以下命令。$image_name 是文件名，$post_id 是帖子ID。

if( $image_name != \'\' && !has_post_thumbnail( $post_id ) ){
  $base = get_stylesheet_directory();
  $imgfile= $base . \'/import/\' . $image_name; // Images are stored inside an imports folder, in the theme directory
  $filename = basename($imgfile);
  $upload_file = wp_upload_bits($filename, null, file_get_contents($imgfile));
  if (!$upload_file[\'error\']) {
    $wp_filetype = wp_check_filetype($filename, null );
    $attachment = array(
      \'post_mime_type\' => $wp_filetype[\'type\'],
      \'post_parent\' => 0,
      \'post_title\' => preg_replace(\'/\\.[^.]+$/\', \'\', $filename),
      \'post_content\' => \'\',
      \'post_status\' => \'inherit\'
    );
    $attachment_id = wp_insert_attachment( $attachment, $upload_file[\'file\'], $post_id );

    if (!is_wp_error($attachment_id)) {
      require_once(ABSPATH . "wp-admin" . \'/includes/image.php\');
      $attachment_data = wp_generate_attachment_metadata( $attachment_id, $upload_file[\'file\'] );
      wp_update_attachment_metadata( $attachment_id,  $attachment_data );
    }
    set_post_thumbnail( $post_id, $attachment_id );

  }
}

如果处理大量图像/帖子，可能会遇到PHP超时。如果是这样，只需再次运行该函数。有一个已经附加的图像检查，所以这应该不是一个问题。