Problem inserting Bytea in postgreSQL

I made this update to update a column in my company table, my column in the database is of type bytea and I am saving an image as a byte [] ... in my company class, I have the variable is of type Byte [] Foto

 public int Update(clsEmpresa E)
  {
     int r = 0;
     string sql = @"update empresa set emp_figura = ?;";

 string connString = String.Format("Server={0};Port={1};User Id={2};Password={3};Database={4};",
 clsConfigBanco.SERVERNAME, clsConfigBanco.PORT, clsConfigBanco.USERNAME, clsConfigBanco.PASSWORD, clsConfigBanco.DATABASENAME);

 using (NpgsqlConnection conexao = new NpgsqlConnection(connString))
 {
     conexao.Open();
     using (NpgsqlCommand cmd = new NpgsqlCommand(sql, conexao))
     {
        NpgsqlParameter param = new NpgsqlParameter("emp_figura", 
        NpgsqlTypes.NpgsqlDbType.Bytea); 
        param.Value = (E.Foto == null ? null : (E.Foto.Length == 0 ? null : E.Foto));
        cmd.Parameters.Add(param);
        r = cmd.ExecuteNonQuery();
     }
     conexao.Close();
 }

     return r;
}

When I run this method the following error occurs ERROR: 42601: syntax error at or near ";" when I do cmd.ExecuteNonQuery;

Would the error be referring to using NpgsqlTypes.NpgsqlDbType.Bytea related to the type of my parameter? What would be the correct way to save this byte array in the database?