Updating string in a picture box

i want to create a little game. Therefore I need some help, because last time I used the Graphics-Library was 10 years ago.

What I plan to do:
I have a picturebox and a grapics object

Graphics g;
void MainThread()
{
    Thread.Sleep(500);
    g.FillRectangle(MyBrushes.White, 0, 0, pbImage.Width, pbImage.Height);
    Graphics gBlank = g;
    while (shown)
    {
        g = gBlank;
        g.DrawString(string.Format("X: {0}/nY: {1}", MouseChords.X, MouseChords.Y), new Font("Arial", 10.0f), MyBrushes.Black, 755,560);
        Thread.Sleep(50);
    }
}

public frmMain()
{
    g = pbImage.CreateGraphics();
    thrMain = new Thread(new ThreadStart(MainThread));
}

I’d like to update the Cursorposition in the down right corner, but as I’m moving the mouse over the control it looks like this.