C sharp - Cargar un imagen de una base de datos postgresql en una columna image de un datagridview

if (cn.State == ConnectionState.Closed)

{

    cn.Open();

    NpgsqlCommand cmd = new NpgsqlCommand("select * from documentos order by iddocumento asc;", cn);

    NpgsqlDataReader dr;

    dr = cmd.ExecuteReader();

    int fila = 0;

    while (dr.Read())

    {

        dtgDocumentos.Rows.Add();

        dtgDocumentos[0, fila].Value = dr[0];

        dtgDocumentos[1, fila].Value = dr[1];

        dtgDocumentos[2, fila].Value = dr[2];

        dtgDocumentos[3, fila].Value = dr[3];

        byte[] imagen = (byte[])dr[4];

        System.IO.MemoryStream ms = new System.IO.MemoryStream(imagen);

        dtgDocumentos[4, fila].value =  Image.FromStream(ms);

        fila++;

    }

    cn.Close();

}

public void cargagrid(System.Windows.Forms.DataGridView DGV, String Instruccion)

{

    FbConnection  fireconex = new FbConnection(conFireb);

    if (fireconex.State == System.Data.ConnectionState.Open) fireconex.Close();

    try {

        fireconex.Open();

        FbDataAdapter DA = new FbDataAdapter(Instruccion, fireconex);

        DataSet DS = new DataSet();

        DA.Fill(DS);

        DGV.DataSource = DS.Tables[0];

        DGV.AllowUserToAddRows = false;

    }

    catch (Exception exdg)

    {

        System.Windows.Forms.MessageBox.Show("Ocurrió un error. " + exdg.Message);

        throw;

    }

}