Are there no existing services that will work for him? If so, I'd be curious what "let other people use them" means. He is going to have people directly linking

Are there no existing services that will work for him? If so, I'd be curious what "let other people use them" means. He is going to have people directly linking to them which will chew up his bandwidth. And if you plan to put controls on that, then you're going down a path of greater development complexity. I recommend Cloudinary or 6px for doing any non-trivial image-based websites. Those services abstract away huge parts of the complexity of storing, manipulating, and delivering images online.