[cd34]

if your browser is not sending a referrer, due to some privacy software or whatever, it will still show the image.

You can comment out that first line:

#RewriteCond %{HTTP_REFERER} !^$ [NC]

and it will block empty referrers. That could block legit surfers that are running privacy software, but, there's always the possibility they won't accept the cookie and you wouldn't get the sale anyhow.