I used this analogy more from the user's perspective (as a user, a gradually sharpening image feels similar to a website with glimmers gradually getting replaced by revealing content). I don't actually know how JPEG is served under the hood (and the spec is too dense for me) so maybe if you explain the point a bit closer I'll be able to follow. I do believe you that the analogy doesn't go all the way.
RSC streams outside-in because that's the general shape of the UI — yes, you might want to prioritize the video, but you have to display the shell around that video first. So "outside-in" is just that common sense — the shell goes first. Other than that, the server will prioritize whatever's ready to be written to the stream — if we're not blocked on IO, we're writing.
The client does some selective prioritization on its own as it receives stuff (e.g. as it loads JS, it will prioritize hydrating the part of the page that you're trying to interact with).
RSC streams outside-in because that's the general shape of the UI — yes, you might want to prioritize the video, but you have to display the shell around that video first. So "outside-in" is just that common sense — the shell goes first. Other than that, the server will prioritize whatever's ready to be written to the stream — if we're not blocked on IO, we're writing.
The client does some selective prioritization on its own as it receives stuff (e.g. as it loads JS, it will prioritize hydrating the part of the page that you're trying to interact with).