That's true. It doesn't really simplify the server implementation, because you still need to basically re-implement WebFinger for clients that don't accept imag

That's true. It doesn't really simplify the server implementation, because you still need to basically re-implement WebFinger for clients that don't accept image/*, but it makes it possible to implement a really simple client that just makes a simple GET for an image resource and either gets it or doesn't, without needing to implement any client logic for WebFinger. I don't think there's really any performance benefit to "one less HTTP request" since you still only even get that case if the client accepts a direct image AND the image is served directly in response to the well known query (instead of serving a redirect to a file store or something where the image really lives), but it might be nice if WebFinger had a method of providing "shortcuts" to resources so this was possible without implementing a new WebFinger-like protocol on a new well-known url.