OpenFileDialog od = new OpenFileDialog();
if (od.ShowDialog(this) == DialogResult.OK)
{
    try
    {
        Image img = Image.FromFile(od.FileName);
        this.pictureBox1.Size = img.Size;
        this.pictureBox1.Image = img;
    }
    catch (Exception ex)
    {
        MessageBox.Show(ex.Message);
    }
}