How Do I Change The Picturebox Image Of From1 From From2

//globally define variable on form2

public static Image image;

//form2 create an image

void CreateBitmap()
        {
            System.Drawing.Bitmap flag = new System.Drawing.Bitmap(50, 50);
            for (int x = 0; x < flag.Height; ++x)
                for (int y = 0; y < flag.Width; ++y)
                    flag.SetPixel(x, y, Color.White);
            for (int x = 0; x < flag.Height; ++x)
                flag.SetPixel(x, x, Color.Red);
            image = flag;
            pictureBox1.Image = flag;
        }

//than on button click call createbitmap method and redirect it to form1

private void button1_Click(object sender, EventArgs e)
        {

            CreateBitmap();
            Form1 f1 = new Form1();
            f1.Show();
        }

paste below code on form1 constructor

pictureBox1.Image = Form2.image;