还有Graphics.FromImage（）那句是干什么的。。。
private Bitmap bits;
private Graphics bitG;
private void pictureBox1_Click(object sender, EventArgs e)
{
bits = new Bitmap(pictureBox1.Width, pictureBox1.Height);
bitG = Graphics.FromImage(bits);
bitG.Clear(Color.White);
pictureBox1.Image = bits;
}
善用搜索，微软的msdn库说得比我们还清楚啦。