lucy24 - 12:50 am on Feb 5, 2013 (gmt 0)
Oh, the image-search preview. They probably count it as Not A Robot in the same way that ordinary Preview (the www-search kind) is Not A Robot. Look at your logs and you should be able to figure out exactly who's doing the requesting and when.
Matter of fact, I'm going to do a bit more experimenting. This will involve setting select images to no-caching-at-all, so I can only hope that both g### and my ISP honor this directive. (Like so much else on the internet, it works on the honor system.)