设置了winform的背景图片随窗体的变化而变化

 private void mainform_Paint(object sender, PaintEventArgs e)
        {
            Graphics g = e.Graphics;
            System.Drawing.Bitmap xzfy = new System.Drawing.Bitmap(Application.StartupPath + "//img/背景.png");
            g.DrawImage(xzfy, this.ClientRectangle);
        }
        private void loadBackImage()
        {
            Bitmap bit = new Bitmap(this.Width, this.Height);
            Graphics g = Graphics.FromImage(bit);
           // g.DrawImage(this.pictureBox1.Image, new Rectangle(0, 0, bit.Width, bit.Height), 0, 0, this.pictureBox1.Image.Width, this.pictureBox1.Image.Height, GraphicsUnit.Pixel);
            this.BackgroundImage = bit;
            g.Dispose();
        }
 private void mainform_Paint(object sender,