Nice! I think you can approximate timing somewhat, by making your web server create the "jpeg" on the fly and send it to the client in timed chunks. The source could even be a webcam, so the "jpeg" would go on forever.
Excellent hack! Should definitely be possible to make an animated gif to jpeg converter. I guess the animation could be slowed a little by repeating frames.
That's basically the server telling the client 'That data I just sent you, well now replace it with this new thing'.
No JavaScript needed, and can work with plain http and jpeg
Easy enough to add a delay() each frame if your server is python/nodejs/PHP/whatever
But this is clever - just smash them together. Low frequency of one image concatenated with high frequency from another. This works surprisingly well!