JonW - 11:48 pm on Jan 16, 2009 (gmt 0)

Agreed referrer isn't perfect.

How about using the image css hack to detect if the user has ever visited the other website.

Also, you could assign each image a token that's only valid for a short time. All other links would have to be served with new token, so it might not work if all the links that are ok are not within your control and on php/asp pages.