Interesting. And it's also weird how bright all shots are. Even ANH looks much less crushed in the shadow detail.
I found one shot with a lot of compression artefacts though: http://ninfo.eu/wp-content/gallery/StarWars/Episode.4/StarWars.Episode.4.39.jpg
So all of them can't be from a digital intermediate.
Maybe some kind of pre-release promo discs?