Who, or what, are the human characters in the original 1977 Hildebrandt Star Wars poster? They are definitely NOT Mark Hamill or Carrie Fisher. Is there any canon information regarding whether or not they were based on specific human models, or just drawn from the artists imagination?
Per Theforce.net, the actors you see in the poster are Mark Hamill and Carrie Fisher, albeit heavily stylised.
The poster you've shown was a "reimagining" (e.g. a copy) of an earlier poster drawn by Tom Jung (see below). The Hildebrand brothers were given the Jung poster but not headshots of Carrie Fisher and Mark Hamill and asked to turn it into a set of colourised movie posters.
"We had a problem with Carrie Fisher," noted Jung, "because they wanted to make her more glamorous. I got my wife to pose for the drawing, and I changed the hairdo and shoved the paint around until I came up with the figure you see now. Carrie Fisher's mother, Debbie Reynolds, loved the poster. She called David Weitzner at Fox's advertising department and asked if she could have the painting, so he asked me to do a duplicate painting, which is now hanging in Carrie Fisher's house. The original painting is at Skywalker Ranch." Cinefantastique Feb. '97 (by Lawrence French)
They hired twins Tim and Greg Hildebrandt, who were well-known fantasy artists at the time, to paint their own take on the Jung design. “The reason they called us is because Tim and I had just done the Lord of the Rings calendar, and we had a fan following.” says Greg Hildebrandt. “We had come through literally overnight for them on a poster for Young Frankenstein. It wasn’t used, but we did it overnight, so they called us and said we need a poster fast.” Star Wars Poster Book