RU.08 I got this 1920x1080 Star Wars wallpaper from themoviedb.org (it is now gone). It seems to be either from a better scan of the posterbook or the same scan with less alteration/smoothing.
It's a shame the posters there seem to max out around 1500px as I'm sure many of them exist at a higher quality. The only "A" poster I have seen with the same colouring and detail as what msycamore posted was a photo of a six sheet but it was so badly folded it was useless for a digital copy.