An Imgur user named "joinyouinthesun" recently posted a whopping 1,138 behind the scenes images from Star Wars, The Empire Strikes Back, and Return of the Jedi. The huge collection shows all manner of different aspects from the filming of the original trilogy, some of which I've never seen before and some of which are just pure awesome—like the image of Sir Alec Guinness on the right.
These images include concept art by Ralph McQuarrie; pre-production, such as the designing of the films; production, like the filming of the climactic lightsaber duels; and post-production, including the creation of the special effects used in the trilogy's battles.
To see all of that and more, check out the full collection of images.