You can read more about his influence in those movies via the link above, but in short, many of the designs were his, including Darth Vader and his breathing apparatus, and his concept art helped get funding from the studios to make A New Hope. He didn't work on the prequel trilogy (he was "out of steam"), but it's interesting to wonder how things might have looked with his involvement.
As a teenager, I checked out The Illustrated Star Wars Universe many times from the library, and if you're interested in Ralph McQuarrie or Star Wars, I highly recommend it. It showcases various planets in a tour guide sort of way, but of course it's worth a look just for the art. My very favorite painting in the whole book comes from the section on Alderaan. Every time I look at this painting I just want to crawl inside it and live there. Doesn't it look so warm and green and peaceful?