no, I mean, for example, when user click at your link, it will open page to upload an image, right?
ok, he chosed the file to upload, pressed upload button.
then, u already have user_id in $user_info['user_id'] variable, no need send it along with the file.
so, in the code, u can load ID like:
$user_id = $user_info['user_id'];
instead of
$user_id = $HTTP_POST_VARS['user_id'];