Star Wars concept art from Ralph McQuarrie. One has to be careful, because a lot of art that is claimed to be pre-film artwork by McQuarrie is not.
This is his concept for the 'droids on Tatooine. Note how far more feminine he makes C-3PO (the look is very Metropolis), and how he perceives R2-D2 to be more mobile, with arms and appurtenances.
Here's his Darth Vader concept art. It was McQuarrie who suggested that Vader wear a mask and have breathing support. NOT GEORGE LUCAS! His work is very Disneyesque, with a mouth designed to look like it was shouting and arches over the eyes to indicate he was angry.
McQuarrie envisioned Tatooine as a planet with two suns, something Lucas had not thought of. McQuarrie wanted it to look incredibly flat and sunbleached, almost blue (in the way extreme sunlight can be). He also designed the look of Luke Skywalker's landspeeder.
McQuarrie also designed the look of the Sand People. It was mimicked almost entirely by the art directors and costumers. McQuarrie saw the banthas as horse-like rams, not the slow-moving, elephantine creatures Lucas created. (That's because Lucas wanted to do massive riding animals, but had no budget to do mechanical creatures. Being in Tunisia, he just used elephants covered in rugs.)
McQuarrie envisioned the Jawas as a sort of Gypsy people, and their sandcrawlers as big front-loading cargo ships on treads. They were lithe craft, not the ponderous and creaky things Lucas created. McQuarrie said that they had to be light, for otherwise they would fall into sand pits or traps. McQuarrie thought that the Jawas should set up camps like carnivals or bazaars, and people would come from miles around to look at their crafts and machines, eat their food, and participate in their games. Lucas saw the Jawas as junk dealers.
McQuarrie later said he drew inspiration for the sandcrawlers from recently-invented cargo ships whose bows could open to receive cargo.
McQuarrie drew heavily on North African architecture for Mos Eisely. Round-topped structures meant sand could not collect on the roofs and cause them to collapse. He liked the idea of minarets as well, and placed freizes of religious text around the top of each building (something he'd seen in photographs of buildings in Chad and Western Sahara).
Lucas saw Mos Eisely as more of a collection of junked cargo ships, cargo containers, slap-dash building, and a few well-built official residences. He rejected the idea that Tatooine would have a culture, religion, architecture, art, etc., and saw it more like a Wild West frontier trading post.
This is one of the most important pieces of concept art McQuarrie did for the film. Although George Lucas' script talked about "X-wing" and "Y-wing" fighters, giving McQuarrie an idea for their design, there was nothing in the script about the look of the TIE fighter. McQuarrie came up with the idea of the cockpit of the TIE fighter by imitating ball-gun turrets from the B-24, a World War II bomber. He used rectangular solar panels for the wings. The design was later changed by ILM, but the basic concept was McQuarrie's.
This was one of the images which convinced 20th Century Fox to fund the motion picture.