cannot draw ellipse on form load in c#

I am trying to draw some ellipse in a picturebox that contains a PNG on a formload in c#. When I execute the code down below, I see my ellipses for half of a second, then I don't see them no more.

When I click on my picturebox, I am able to draw an ellipse however, when I minimize the form, they don't appear no more.

I've read that you shouldn't put your drawing code in the formload but rather in the OnPaint method, which is what I did. I don't know what to try anymore. thank you. (Be aware that I've left some code commented to show what I've tried).

public partial class FormParterre : Form
{
    Graphics g;
    float circleSize = 15;
    //Brushes rouge = new Brushes (Brushes.Red);

    ReservationBilletSiegeDAO reservationBilletSiegeDAO = new ReservationBilletSiegeDAO();
    SiegeDAO siegeDAO = new SiegeDAO();

    List <Siege> sieges;
    List<ReservationBilletSiege> rbs;
    ReservationBillet reservationBillet = new ReservationBillet();
    ReservationBilletSiege reservationBilletSiege;

    SolidBrush semiTransBrush;


    public FormParterre()
    {
        InitializeComponent();
        pictureBox1.Image = new Bitmap("./parterre.png");
        g = pictureBox1.CreateGraphics();


    }

    public FormParterre(ReservationBillet rb)
    {
        reservationBillet = rb;
        pictureBox1.Image = new Bitmap("./parterre.png");
        g = pictureBox1.CreateGraphics();



        InitializeComponent();
    }

    protected override void OnPaintBackground(PaintEventArgs e)
    {
        // do nothing! prevents flicker
    }

    protected override void OnPaint(PaintEventArgs e)
    {

        sieges = siegeDAO.readAll();

        rbs = reservationBilletSiegeDAO.readAll();

        foreach (ReservationBilletSiege reservationBilletSiegeTMP in rbs)
        {
            Console.WriteLine(reservationBilletSiegeTMP.toString());
            int x = siegeDAO.read(reservationBilletSiegeTMP.idSiege).xValeur;
            int y = siegeDAO.read(reservationBilletSiegeTMP.idSiege).yValeur;
            float xx = (float)x;
            float yy = (float)y; 
            Console.WriteLine("le x: " + xx);
            Console.WriteLine("le y: " + yy);


              /*e.Graphics.FillRectangle(new SolidBrush(BackColor), e.ClipRectangle);*/
            g.FillEllipse(new SolidBrush(Color.FromArgb(128, 0, 0, 255)), xx - circleSize / 2, yy - circleSize / 2, circleSize,                                     circleSize);
        }

    }

    private void pictureBox1_Click(object sender, EventArgs e)
    {  
        MouseEventArgs me = (MouseEventArgs)e;
        txtX.Text = me.X.ToString();
        txtY.Text = me.Y.ToString();

        Console.WriteLine("click"); 

        g.FillEllipse(new SolidBrush(Color.FromArgb(128, 0, 0, 255)), float.Parse(txtX.Text) - circleSize / 2, float.Parse(txtY.Text) - circleSize / 2, circleSize, circleSize);





    }

    private void FormParterre_Shown(object sender, EventArgs e)
    {

    }

    private void FormParterre_Load(object sender, EventArgs e)
    {

        /*sieges = siegeDAO.readAll();
        //semiTransBrush = new SolidBrush(Color.FromArgb(128, 0, 0, 255));



        rbs = reservationBilletSiegeDAO.readAll();

        foreach (ReservationBilletSiege reservationBilletSiegeTMP in rbs)
        {
            Console.WriteLine(reservationBilletSiegeTMP.toString());
            int x = siegeDAO.read(reservationBilletSiegeTMP.idSiege).xValeur;
            int y = siegeDAO.read(reservationBilletSiegeTMP.idSiege).yValeur;
            float xx = (float)x;
            float yy = (float)y; 
            Console.WriteLine("le x: " + xx);
            Console.WriteLine("le y: " + yy);

            g.FillEllipse(new SolidBrush(Color.FromArgb(128, 0, 0, 255)), xx - circleSize / 2, yy - circleSize / 2, circleSize, circleSize);
        }*/
    }


}

You need to paint in the OnPaint method using the PaintEventArgs.Graphics property which is passed to you. If you want to paint on the PictureBox you can try subscribing to its Paint event and painting there.

This works for me:

public void Form1()
{
    InitializeComponent();

    pictureBox1.Paint += pictureBox1_Paint;
}

private void pictureBox1_Paint(object sender, PaintEventArgs e)
{
    e.Graphics.FillEllipse(Brushes.Red, pictureBox1.ClientRectangle);
}