And how will that continue to work when if it goes beyond Twitter, G+, and FB? Most browsers send a referrer in HTTP GET for the img tag. It works now for those because google and facebook grok the html, pull out the image, and cache it.
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[1] Though, rather unfortunately, most browsers do not seem to send reasonable Accept headers. See https://developer.mozilla.org/en-US/docs/HTTP/Content_negoti..., "Default Values" and "Values for an image".