By 1975, the films 'visual identity' had already been somewhat cemented prior to the film getting the green-light, mainly due to the amazing illustrations and designs created by the legendary Ralph McQuarrie. for which he had been commissioned by George Lucas. According to his Wikipedia page...
McQuarrie's concept paintings were instrumental in helping Lucas to win approval from 20th Century Fox; armed with vivid illustrations of his planned movie, Lucas was able to convince Fox executives to take a gamble and fund his Star Wars project. Despite their scepticism, it became a huge success upon release in 1977. Among McQuarrie's Star Wars portfolio were concept paintings depicting scenes on the planet Tatooine, inside the Mos Eisley cantina, inside the Death Star and on the moon of Yavin. During filming, Lucas ensured that many shots reproduced McQuarrie's paintings exactly, such was his esteem for McQuarrie's work.[7] McQuarrie has said of his work on Star Wars, "I thought I had the best job that an artist ever had on a film, and I had never worked on a feature film before. ... I still get fan mail — people wondering if I worked on Episode I or just wanting to have my autograph."
Examples include...

...and this...

...and perhaps most famously...
