(I'll just cut-and-paste my post from TFN which, in turn, was mostly just cut-and-pasted from a comment I left at McQuarrie's Facebook page.)
Here's the post I left at his Facebook page:
I always get the great science fiction "sense of wonder" from Ralph McQuarrie's beautiful paintings. This is especially true with Star Wars, but I see now that he did so much else beside that. Most notably, it was clearly McQuarrie who was chiefly responsible for Darth Vader's appearance. I think he was a wonderful creator and I hope he's in Heaven! I'll never forget the feelings of awe I had reading making of Star Wars books in high school when I saw his paintings.
I dormed at a high school, and I remember having to sit in this study hall for two hours every night while this creepy dean of discipline hovered over us. In that time, I remember just staring at McQuarrie's beautiful paintings in these two books:
Just getting lost in them. As detailed in, 'The Secret History of Star Wars', Ralph McQuarrie is almost entirely to credit with the design of Darth Vader.
From the above quote it sounds as if McQuarrie's masked drawing inspired Lucas' descriptions, and not the other way around—given that Lucas liked to work this way on the sequels and prequels, this is unsurprising. In tact, McQuarrie began sketching in November 1974210—when Lucas was still writing. He elaborates:
"George came along in about a week with a little bundle ot suit! he'd gotten out ot old science-Fiction magazines ot the 1930s and material like that. George also supplied some books on Japanese medieval stuff... George had mentioned him having to wear a helmet like a Japanese medieval warrior, one ot those big flared-out helmets, and I made it somewhere between that and a German World War II helmet. In probably one day, I made all the drawings that pretty much defined Darth Vader. I was moving veiy tast and didn't have all week to tool around with Darth Vader—I had lots ot other things to work on... [George] was veiy happy it you came up with some ideas that was completely different. George didn't envision Darth Vader with a mask—he said he might have his face covered with black silk. But I got worried for Vader's health, because he has to transfer to another spacecraft through outer space with stormtroopers who had armored space suits...George said, 'Well, all right, give him some kind ot breath mask,"—which he wore through all three films."