That's the cover of Star Wars #53, from 1981. You may recognize the art style as belonging to Walt Simonson, who posted it on Facebook, but there's an interesting story behind it.
Marvel had a huge success with their Conan comic, so they tried to recapture that glory by licensing another Edgar Rice Burroughs character for John Carter, Warlord of Mars. That comic only lasted a few years, and when it ended, there was still one issue left unpublished - a fill-in story drawn by Carmine Infantino.
Since that story was already drawn and paid for, Marvel didn't want to let it go to waste, so they had Chris Claremont and Walt Simonson rework the pages as a Star Wars story (and the reason they tapped Simonson was probably that he'd earlier turned an issue of Tarzan into Battlestar Galactica). The one-issue Warlord of Mars story became a Star Wars two-parter, with some clear John Carter parallels:
The giant Stormtroopers on the cover aren't a mistake - they really were that big in the story, because they had to be drawn over existing aliens.
Readers at the time noticed the connection to John Carter, but the letters column in issue #59 simply claimed that the issues had been "inspired" by the Warlord of Mars, without revealing the true connection. How about that!