Gregor does a lot of thinking as he attempts to get himself out of bed after the change--he thinks of how he is the only one who really works in the family--no one else earns money. His mother works in the home, his father is sickly, and his sister plays the violin. Perhaps the first symbol, then, is the fact that he is "changed" into a roach-like being (it never says exactly what kind of bug he is, but he is very roach-like...yuck!) He is working for everyone else and he loathes/resents it, so it is fitting that he is morphed into a creature that most people loathe.
Another symbol is the picture of the lady on the wall. His sister removes all the furniture in the room so Gregor will have more room to move and not hurt himself. He fights to keep the picture as it is symbolic of the only human elements left to him and of him.
Gregor himself is a symbol. Before he "changed" he was the only one working and making a living. After the change, all the members of the family become workers and in a sense, valued. Had he not gone through his metamorphosis, the family may never have discovered the hidden talents and worth they had simply because they had Gregor to depend upon for all they needed.
Kafka is sending a message of hidden worth. It's great to have people to depend on, but never give up on your own dreams to do something wonderful with your life. Also, working in a job you hate will kill you.
I think this book is saturated with metarphors, similes and images. Symbols are part of the group,the apple first of all symbolises the begining of the end like the Bible story about Eve eating the apple it showed that man would die just like gregor died when he was hit by his own father. The window which was the only on in the room or which was Gregors fav one shows the power in numerology and this is proven by the number three that occurs constantly in this amazing story he died at the hr three and the door symbolises the isolation being shut in the key shows imprisonment and this is how we get the idea of isolation