Complex sentence: Charles W. Duncan (1887--1970) was an American avant-garde painter in the circle of artists that gathered around the photographer and art promoter Alfred Stieglitz, is now known primarily as the subject of one of Charles Demuth's famous poster portraits.
Split into simpler sentences: Charles W. Duncan (1887--1970) was an American avant-garde painter in the circle of artists that gathered around the photographer and art promoter Alfred Stieglitz. He is now known primarily as the subject of one of Charles Demuth's famous poster portraits.