Create a property on Form2 which sets the Image on it's PictureBox:
public Image Image
{
get { return myPictrueBox.Image; }
set { myPictureBox.Image = value; }
}
Then set it via the instance of Form2 that you create in Form1:
Form2 f = new Form2();
f.Image = myForm1PictureBox.Image;
f.ShowDialog();