Clear image on picturebox

How can I clear draw image on picturebox? The following doesn't help me:

pictbox.Image = null;
pictbox.Invalidate();

Please help.

EDIT

private void pictbox_Paint(object sender, PaintEventArgs e) 
{ 
     Graphics g = e.Graphics; 
     vl.Draw(g, ref tran.realListForInsert); 
} 

public void Draw(Graphics g, ref List<double> arr) 
{ 
    g.DrawRectangle(new Pen(Brushes.Red, 3), nodeArr[Convert.ToInt32(templstName)].pict.Location.X, nodeArr[Convert.ToInt32(templstName)].pict.Location.Y, 25, 25); 
    g.DrawRectangle(new Pen(Brushes.Green, 3), nodeArr[Convert.ToInt32(templstArgName)].pict.Location.X, nodeArr[Convert.ToInt32(templstArgName)].pict.Location.Y, 25, 25); 
    nodeArr[Convert.ToInt32(templstName)].text.Text = arr[Convert.ToInt32(templstArgName)].ToString(); 
    arr[Convert.ToInt32(templstName)] = arr[Convert.ToInt32(templstArgName)]; 
}