The first posters released for Revenge of the Jedi are fairly famous, mostly for not matching the title of the final film.
However, from the first time I saw it, what struck me was that, regardless of what the Star Wars, Episode VI was going to be called, what it showed on the poster was not from it. The fighting between Luke Skywalker and Darth Vader is very obviously adapted from their duel in The Empire Strikes Back. You can see the distinctive steps of the carbon freezing chamber under their feet, and Vader is wielding his lightsaber one handed, something that George Lucas was vocally unhappy about and which he made sure did not recur in Return of the Jedi. Indeed, here is the very frame from The Empire Strikes Back that shows Luke posed as on the poster, although Vader is positioned somewhat differently.
The poster was created by the graphic artist Drew Struzan, who later created posters for subsequent Star Wars as well as rereleases of the original trilogy. Struzan’s specialty appears to be making collages of images, where the individual elements are drawn based on photographs (or other preexisting images) and combined to make an evocative image. That is just what The Revenge of the Jedi poster shows: two drawings of Vader and one of Luke, all, given their precise detailing, clearly based on photographs or film stills.
But why was Struzan working from material from The Empire Strikes Back? Was it that no stills from Return of the Jedi were available to him? (And, if so, why? Was it a matter of secrecy? Principal photography for Return of the Jedi ran from January to May, 1982, so it seems like there ought to have been something they could have given the artist at the time he was hired.) And did he chose the Cloud City battle scene himself, or was he instructed to work from that? (Then by whom?) Finally, was there a reason he included the steps Luke is standing on (which were, for me, what made it most obvious where the original image came from)?