At last it arrested her -- and she beheld a striking resemblance of Mr. Darcy, with such a smile over the face as she remembered to have sometimes seen when he looked at her. She stood several minutes before the picture in earnest contemplation, and returned to it again before they quitted the gallery. Mrs. Reynolds informed them that it had been taken in his father's life time. Chap 43

This is a picture of a younger, perhaps more carefree, Darcy. Viewing the portrait is a turining point for Elizabeth

There was certainly at this moment, in Elizabeth's mind, a more gentle sensation towards the original than she had ever felt in the height of their acquaintance. The commendation bestowed on him by Mrs. Reynolds was of no trifling nature. What praise is more valuable than the praise of an intelligent servant? As a brother, a landlord, a master, she considered how many people's happiness were in his guardianship! -- how much of pleasure or pain it was in his power to bestow! -- how much of good or evil must be done by him! Every idea that had been brought forward by the housekeeper was favourable to his character, and as she stood before the canvas, on which he was represented, and fixed his eyes upon herself, she thought of his regard with a deeper sentiment of gratitude than it had ever raised before; she remembered its warmth, and softened its impropriety of expression.

Visiting Pemberley seems to wipe the slate clean for Darcy with Elizabeth. The just minutes after seeing the portrait she runs into him.