I may be mistaken but I thought Safari's "activity" view showed downloads made by Flash applets. So you just get the URLs for every image served up right on a silver platter. I'd assume it works the same in Chrome.

You can just as easily deter casual image saving by doing what some folks do and put a clear image layer above the actual image, so the browser prompts the user to save a 1px clear png instead. If they know enough to view the source or use a DOM inspector you can't really stop them anyway.