using (SqlCommand command = Connection.CreateCommand())
            {
                command.CommandText = "INSERT INTO forum.dbo.Messages(parent_id, date_time, author, body, topic_id, image_url) " +
                                      "VALUES (@parent_id, @date_time, @author, @body, @topic_id, @image_url)";

                command.Parameters.Add("@parent_id", SqlDbType.Int).Value = msg.ParentId;
...

// Может есть стандартный способ?
                if (!String.IsNullOrEmpty(msg.ImageUrl))
                {
                    command.Parameters.Add("@image_url", SqlDbType.NVarChar).Value = msg.ImageUrl;
                }
                else
                {
                    command.Parameters.Add("@image_url", SqlDbType.NVarChar).Value = DBNull.Value;
                }

                command.ExecuteNonQuery();
            }