Short version: If I decide to reject access to some URI because a client doesn't supply a valid Referer HTTP header, what http status do I response?
Long: Suppose I wrote a blog post with very nice photos. I want my visitors share with their friends by using a link to this blog post, but am a little sad if my visitors share with their friends with a link directly to the image files bypassing my blog post. Besides persuading in words like “please share with your friends with a link to this blog post”, what is a polite way to indicate that the images files are better accessed with corresponding HTTP Referer header by using a certain http status?