Accessing Form3.pictureBox1 out of Form1
Form3.pictureBox1.Image = MyImage;

Form3 frm3 = new Form3();
frm3.pictureBox1.Image = Image.FromFile("ImageName");
frm3.Show();

string a;

static void Main()
{
    Application.EnableVisualStyles();
    Application.SetCompatibleTextRenderingDefault(false);
    Application.Run(new Form1());
}

static void Main()
{
    Application.EnableVisualStyles();
    Application.SetCompatibleTextRenderingDefault(false);

    Form1 f = new Form1();
    Application.Run(f);
}

Form2 frm2 = new Form2();
frm2.show();
frm2.pictureBox1.Image = "MyImage";