levo - 12:11 pm on Feb 7, 2013 (gmt 0)
They might have a little goldmine on their hands and not even have realised it:-)
They are using 302 redirect. No referer and/or other domains redirect to watermarked image (with a random number in URL).
Scratch that, if the UA for page request is Googlebot, they serve img links without CDN. When Googlebot(-image?) requests the image without the cdn subdomain, they serve the actual image, if a visitor (any other UA) requests the image without the cdn subdomain, they serve the watermarked one. If visitors clicks the result and goes to the website, embedded image URL includes the CDN subdomain.