Symbolism, Imagery, Allegory
Boy, is this portrait ugly. But that's not the point. Well, not entirely, anyway.
Camille's portrait plays a pivotal role in Laurent's breakdown. When he first paints Camille's portrait, the description of Camille's deformed face foreshadows what Laurent will see when he sees Camille's drowned body in the morgue. Dun dun dun.
After Camille's death, Laurent imagines that his portrait comes to life. Zola insists that Laurent's fears aren't manifestations of his moral guilt, but we're not convinced.
Laurent also tries to rediscover himself as an artist, post-Camille. But he finds himself unable to draw anything but Camille's face. In any other novel, Laurent's compulsion to sketch the drowned man's head would be an indication that his conscience is troubling him.
So we're pretty sure that's at least part of what's going on here, no matter what Zola has to say.