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