The portrait of Will's grandmother comes up again and again. Dorothea associates it with Will, because it looks like him, but it could also be seen as a symbol for unhappiness in marriage. It seems to work both ways. Early on in the novel, the narrator says outright that Dorothea
felt a new companionship with it, as if it had an ear for her and could see how she was looking at it. Here was a woman who had known some difficulty about marriage. Nay, the colours deepened, the lips and chin seemed to get larger, the hair and eyes seemed to be sending out light, the face was masculine and beamed on her with that full gaze. (3.28.5)
Dorothea feels "companionship" with the painting itself, and imagines that the woman in it can listen sympathetically to her trouble because she had had problems, too. But then she starts to imagine that it's actually an image of Will Ladislaw, who is the only person who has ever listened to and understood her. The association of the painting with Will Ladislaw, and with harsh judgments more generally, continues throughout the novel. Towards the end of the novel, Dorothea openly caresses the miniature painting: "she took the little oval picture in her palm and made a bed for it there, and leaned her cheek upon it, as if that would soothe the creature who had suffered unjust condemnation" (6.55.2). It's not clear whether the "creature" she's trying to soothe is Will Ladislaw or his grandmother, or maybe it just stands in for all people who are judged unfairly.
Dorothea doesn't just associate the portrait with Will Ladislaw because of the family resemblance. Will's grandmother gave up the Casaubon family fortune to be with the man she loved (also named Ladislaw, of course). Dorothea's interest in Aunt Julia and her portrait could also be seen as foreshadowing her eventual decision to give up the Casaubon family fortune to marry her Ladislaw. So the portrait could be doing something more complicated than the simple association with Will Ladislaw, or with injustice – it could be seen as a symbol of the ways that history repeats itself.