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

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.)