Unfortunately, there is no way to fully prevent someone from downloading your property photos once they are uploaded to the internet. While you can stop someone from right-clicking on a photo to download it, there are numerous other ways someone can get copies - including using the print-screen button, viewing the page source, or various online tools that can easily extract photos.
Another point to remember is that listings are very likely sent to various property portals, which creates additional entry-points for someone to find and download photos.