But that header is valid per client that is using sunshine.social, isn't that so? That means that every client that looks at my profile will fetch the image at

But that header is valid per client that is using sunshine.social, isn't that so? That means that every client that looks at my profile will fetch the image at least once. The images is fetched from IP address 104.131.188.4, and it is that host – not the client – that should cache the image and only download a new copy if If-Modified-Since or eTag indicates that the image has been changed.