The picture in the background of Whistler's Mother is an etching of his earlier work, the Black Lion Wharf:
Whistler originally was going to have a younger model pose, but she fell ill. So, he turned to his mother for a model. She is sitting in the picture, because she is old and frail. Using different types of brush strokes, Whistler created both the detail in her face, hat, lace and hands, and the essence of her clothes, room and curtain.
This is supposedly the 4th most recognizable work of art.