It can't be open like that "just use our URL" because it will bring many free-riders and shutdown your server. But it also can't be a closed system with priva

It can't be open like that "just use our URL" because it will bring many free-riders and shutdown your server. But it also can't be a closed system with private keys and all that, because the hassle is too enormous -- the user will need a server to get the key for each image and so on. I have the impression that all services of this kind suffer from the above dual-problem.