Food, the picture in Gregor's room and his father's uniform are symbols in "The Metamorphosis." Kafka uses these symbols to add depth and insight into Gregor's character.
Food is a symbol because it is necessary for survival. Gregor's tastes as a bug change, and in the beginning of the story, his sister attempts to feed him by leaving a variety of food in his room. Eventually she stops leaving food for him, leaving him without sustenance. Gregor is left to watch as others eat, while he literally starves. When the family becomes disinterested in feeding and caring for him, it symbolizes their acceptance of his eventual death.
The picture hanging in Gregor's room is a symbol that connects Gregor to his past. It is something he selected and enjoyed looking at when he was human and looking at it in his bug state helps him feel more human.
Gregor's father's uniform is symbolic because it affects how Gregor perceives his father. When he first sees his father in the uniform, he feels respect for him. However, as time passes, he realizes his father sleeps in the uniform, causing it to become soiled. The messy uniform causes Gregor to return to his previous lackluster attitude toward his father.