private Form f1;
    private void pictureBox1_Click(object sender, EventArgs e)
    {
        f1 = new Form2();
        f1.TopLevel = false;
        this.panel1.Controls.Add(f1);
        f1.Dock = DockStyle.Fill;
        f1.Show();
    }

    private Form f2;
    private void pictureBox2_Click(object sender, EventArgs e)
    {
        f2 = new Form3();
        f2.TopLevel = false;
        this.panel1.Controls.Add(f2);
        f2.Dock = DockStyle.Fill;
        f2.Show();
    }