The imagery in the poem "Felix Randal" serves to convey the great loss the speaker feels regarding the death of a young man with great talents.
In many ways, the imagery reveals a celebration of Felix Randal's life. He was once a skilled farrier, or horseshoe maker, who worked on "the great grey drayhorse" to create beautiful shoes. Felix Randal was once healthy and strong, "boisterous" in his younger years. He was "big-boned" and "hardy-handsome," a portrait of health and vigor as he engaged in physically strenuous labors to earn a living.
Yet Felix Randal has been utterly transformed because of an illness; the imagery conveys a much different portrait of his final days. Even this once tenacious man could not endure the pain of his fatal illness. His failing health tortured him, causing him to cry out until he lost all sense of "reason." A pointedly short sentence reveals the impact of his illness: "Sickness broke him." And because of this, the speaker tried to intervene, bringing oils to anoint Felix Randal's wounded and failing body.
The speaker reveals that he never could have imagined that Felix Randal could have been so broken by illness back in the days when he was young and strong. Yet the imagery reveals that even the strongest of men eventually succumb to death and to all of the physical pain that is innate in the process of dying.