It is possible, but difficult. You would have to use the function `getCustomImage()` rather than `getNextImageURL()` to get the URL for your images on the image.php page. The last parameter of this function will control whether the image gets a watermark or not.
May I bounce this sort-of request that it would be nice if ZenPhoto allowed us to select which albums should use which plugins, like watermarking, paypal, etc.
