Some production paintings McQuarrie had done for Hal Barwood and Matthew Robbins brought him to the attention of director George Lucas in late 1975. Very soon after, they began discussing production paintings for STAR WARS. Lucas suggested that McQuarrie approach the work from the point of view of “ideal” portrayals rather that feel restricted by what could actually be achieved in filming the situations represented in the art.
The first four of five paintings had been done when STAR WARS was still in the development stage trhough Twentieth Century-Fox. George Lucas felt that McQuarrie’s paintings would not only be of interest to Twentieth, but, by helping them to visualize his ideas, would also dissolve any hesitation on their part to go ahead with making the film.
Wikipedia’s entry on Ralph McQuarrie.