@Rutger Verhaar | Adaptfy Firstly, if the customer can view the image, then they are already downloading it to their computers. While there may be deterrents, there are no real ways to prevent users from downloading images.
As for serving these images with authentication, Azure Functions are a great option. You can even leverage the Authentication / Authorization Feature which setups OAuth2.0 Code Grant Flow for you with no code. Users will be redirected to login automatically and back to the function app URI once done.