Any image displayed on the screen is technically downloaded into the Web browser's cache. If there is no watermark at all on the image when displayed, then a separate system needs to determine if the request is a regular http access or a download access (which I don't think there is currently a way to do that behind the scenes. You would probably have to implement a javascript to detect that). From there, it would have to load the watermark onto it. If several hundred people are doing that at once, it can bog down the server rather quickly. An actual overlay (like how xenforo does for like reporting a post), that would be a waste of resources for both user and server, especially since it can be disabled by disabling javascript. In the end, only one solution would be proper: upload image, keep a copy for the owner (person who uploaded the image), and then render a watermarked copy to display on the forum.