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_SizeChanged(object sender, EventArgs e)

        {

            loadBackImage();

        }

 private void mainform_Load(object sender, EventArgs e)

        {

            loadBackImage();

        }