byte[] img = null;
                FileStream fs = new FileStream(imgLoc, FileMode.Open, FileAccess.Read);
                BinaryReader br = new BinaryReader(fs);
                img = br.ReadBytes((int)fs.Length);
                string cmd = string.Format("insert into estudiantes_fotos values ('{0}','{1}')",
                    codigo_estudiante.Text.Trim(), img);

string cmd = "select * from estudiantes_fotos where codigo_estudiante= '" + codigo_estudiante.Text.Trim() + "' ";

            if (conn.State != ConnectionState.Open)
                conn.Open();

            command = new NpgsqlCommand(cmd, conn);
            NpgsqlDataReader reader = command.ExecuteReader();
            reader.Read();
            if (reader.HasRows)
            {
                byte[] img = (byte[])(reader[1]);
                MemoryStream ms = new MemoryStream(img);
                foto_estudiante.Image = Image.FromStream(ms);

            }