I don't believe you are mistaken. I'm not an expert on all things Star Wars but this image is campaign art by Daniel Goozee.
Appears it was never used as an official movie poster but was used in a print ad.
Original Star Wars unused painting by Dan Goozee for a Star Wars promo poster
This painting is a varation of the final art used for an advertising campaign art from the original 1977 theatrical release of Star Wars was painted by movie poster veteran Daniel Goozee (Moonraker, Octopussy, Clash of the Titans, Superman IV).
The swashbuckling image, of Luke and Leia swinging on a rope, gun blazing, to make their escape from stormtroppers aboard the Death Star, appeared in at least one newspaper ad, but was not widely seen; in fact, apparently the artist himself was unaware that it had actually been used in the advertising at all, since it was just one of many pieces commissioned from various artists by Fox's publicity machine back in the 70s. The concept art is by Dan Gooze with design work by Tony Seiniger and Associates. Its army of stormtroopers on the bottom seems reminiscent of posters for biblical epics.
Goozee is a former Walt Disney "Imagineer" and fine artist who also painted a handful of sci-fi "originals" that were marketed as posters in the back of Famous Monsters and other Warren magazines during the height of the post-Star Wars sci-fi mania. This art was not published in The Art of Star Wars book, but its variant without C-3PO and R2-D2. Dan Goozee designed the below Star Wars poster in November 1977 for the studio. It was never used as an official movie poster but was used in a print ad.
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