Invalidate picturebox with rectangle on

Hello.

I have a simple picturebox where there is a image loaded in

Im drawing a rectangle on the picturebox and when I press a button I want to invalidate it (remove the rectangle), but it doesnt seem to work

The rectangle is drawn fine and the image loads up to, but when I press my button nothing happens

Here is the code im using:
        <sub>private void drawOnPic()
        {
            // Attach grapich to picturebox
            Graphics g = Graphics.FromImage(pictureBox1.Image);

            // Create a new pen that we shall use for drawing the line
            Pen PenStyle = new Pen(Color.Red, 1);

            // Draw a 50x50 pixels rectangle (x, y, width, hight)
            g.DrawRectangle(PenStyle, 20, 20, 50, 50);
        }

        private void button1_Click(object sender, EventArgs e)
        {
            pictureBox1.Invalidate();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            drawOnPic();
        }</sub>