The Man in Black fled across the desert and the Gunslinger followed. The second half of the first line in the first book. The 2 characters couldn't be in the same painting because it's not until they are out of the desert that they come within sight of each other, so he split the sentence into 2 paintings.
Michael has said that this is the closest to what he thinks Roland looks like - dogged, grim, with an indomitable sense of mission.