可以参考here

但是你正在谈论Gravity表单插件中的大量定制，就像你说的，你自己不是程序员，所以你最好在他们的论坛中打开一张支持票，他们可能会提供帮助。

如果你决定自己做，也可以看看这段代码

    $upload_dir = wp_upload_dir();
$image_data = file_get_contents($image_url);
$filename = basename($image_url);
if(wp_mkdir_p($upload_dir[\'path\']))
    $file = $upload_dir[\'path\'] . \'/\' . $filename;
else
    $file = $upload_dir[\'basedir\'] . \'/\' . $filename;
file_put_contents($file, $image_data);

$wp_filetype = wp_check_filetype($filename, null );
$attachment = array(
    \'post_mime_type\' => $wp_filetype[\'type\'],
    \'post_title\' => sanitize_file_name($filename),
    \'post_content\' => \'\',
    \'post_status\' => \'inherit\'
);
$attach_id = wp_insert_attachment( $attachment, $file, $post_id );
require_once(ABSPATH . \'wp-admin/includes/image.php\');
$attach_data = wp_generate_attachment_metadata( $attach_id, $file );
wp_update_attachment_metadata( $attach_id, $attach_data );

set_post_thumbnail( $post_id, $attach_id );