请问如何在子窗体中修改主窗体的pictureBox的图片？

using System.Drawing;
using System.Windows.Forms;

namespace Test1
{
    public partial class Form1 : Form
    {
        public static PictureBox pb;
        public Form1()
        {
            InitializeComponent();

            this.StartPosition = FormStartPosition.CenterScreen;

            pb = new PictureBox()
            {
                Size = new Size(250, 250),
                Location = new Point(10, 10),
                Image = Image.FromFile("3.png"),
            };
            this.Controls.Add(pb);

            Load += (s, e) => {
                new Form2().Show();
            };
        }
    }
}

using System.Drawing;
using System.Windows.Forms;

namespace Test1
{
    public partial class Form2 : Form
    {
        public Form2()
        {
            InitializeComponent();

            this.StartPosition = FormStartPosition.Manual;
            this.Location = new Point(0, 0);

            Button btn = new Button()
            {
                Text = "change!",
                Location = new Point(10, 10)
            };
            btn.Click += (s, e) =>
            {
                Form1.pb.Image = Image.FromFile("11.gif");
            };
            this.Controls.Add(btn);
        }
    }
}