JPEG already supports multiple images per file. If you add EXIF data with manifest information with web rendering hints at the head, your thumbnail in the next frame, and your full-sized image after, you'd be on the track to awesomesauce.

(I'm sure someone has already implemented something better than this, but I am lazy.)

On that note it would be interesting to see if the EXIF data is typically included at the start or end of JPEG files, or if it varies via compressor.

In EXIF, whether applied to JPEG or TIFF, the header comes first, then the thumbnail if present, then the primary image. Exactly as you'd want (which is more than can be said for much of EXIF, e.g. that it lacks time zone encodings).

Then it would still be even faster in total if you do the same thing with progressively encoded images.