glitterball - 7:32 am on Apr 25, 2013 (gmt 0)
The script (.htaccess) works like this:
We check the HTTP_REFERER to see if it originated on our site. If not, we present a watermarked image.
Having actually tried to implement this script, I can see that it's not going to work for us.
Google seem to have done everything in their power to stop webmasters from preventing hot-linking. Why use an iframe, if not to bypass anti-hotlinking scripts?
The only way seems to be to present Google with a modified url of the image, so that we can watermark that one. Since this could be interpreted as cloaking and get us banned, I'm not going to implement it.