This may be old news to everyone else, but I just ran a couple of tests and an image preview referral URL from Google is exactly the same as if I click the link.
So could it be possible the zombies never make it to your website... they simply look at the preview and move on down the list.
Foreign / junk traffic referrals could be image preview on the likes of Google HK, which we all know ranks some UK site quite highly (often higher than on Google UK!).
Apologies if this has already been covered, but I could not see any mention of it.