What is the name of a big white flower that blooms from a bush with big leaves and pastel green stems?

This bush that they are growing on is huge and has numerous flowers on it. No one in my family can think of what they are. The best image I have is this http://www.paintinghere.com/UploadPic/Georgia%20O%27Keeffe/big/White%20Flower.jpg if anyone can help me out it would be great.