keyplyr - 5:24 am on Feb 4, 2013 (gmt 0)
Google Preview is an entirely different issue.* What if your site depends on visitors who need sites-with-pictures-of-X? Ordinary humans don't use Preview for that; they use image search.
I stop Google from getting my images. Google Preview shows a snapshot of my pages (including the images that are on that page) to the right of my listing in the SERP, so obviously it doesn't need to index my image files in order to produce the snapshot.
Except most users don't know what a referrer is and barely understand a COOKIE. The mass populations often doesn't even know what an Internet Explorer or a Firefox is and thinks their Google start page is the internet.
That scenario is changing. All browsers now have a private browsing setting (no referrer) or a stealth mode (same thing) in their tools, besides the employees behind firewalls, military, libraries, univ, internet cafes, several mobile networks, etc that don't send referrers.
Maybe your site looks good without images displaying for the visitor. I've seen my site without them, it looked awful.
I've been blocking images for blank referrers for many years without any complaints.
They won't complain, they just won't come back to a site that looks like that. I've come across plenty of broken sites. Unless I know the owner personally, I've never complained or reported it. Who knows, maybe they want it to look like that - LOL
Got an early class in the morning :)