Few months ago I did an addon for one of my clients, something similar to what smokey suggested, for one of my clients. Basicaly, it rotates image folder every hour (can me more or less frequent).
F.x. That's the current image link
http://static01.vapshare.com/f868ed4...54/126835.jpeg
You may check it out at
http://www.vapshare.com/126835/40791...ottest105.html
Far more effective than htaccess+referrer
