how to insert image into the database

Use the BYTEA type which is an array of bytes. As for the Lua language, I do not know how to tell you, but in C #, just pass byte[] as a parameter normally.

Tip: Prefer to use separate table of images from the records table, so you can insert multiple images into the same record.

Example in C # + ODBC:

byte[] Imagem = (arquivo de imagem em bytes);

OdbcCommand cmd = new OdbcCommand(/*string de conexão*/);
cmd.CommandText = @"INSERT INTO [Tabela]
(coluna_bytea) VALUES (?);";

OdbcParameter[] paramC = new OdbcParameter[1];
paramC[0] = new OdbcParameter("coluna_bytea", OdbcType.Binary);
paramC[0].Value = ((Imagem == null || Imagem.Length == 0 )? null : Imagem);

cmd.ExecuteNonQuery();

To store the photo, who does this is the language of BackEnd, eg PHP. To send ...

http = require("socket.http")

ltn12 = require ("ltn12")

http.request{
    url = "SEU-SERVIDOR/ARQUIVO.PHP",
    method = "POST",
    headers = {
        ["Content-Type"] =  "multipart/form-data",
        ["Content-Length"] = sizeOfFile
    },
    source = ltn12.source.file(io.open(pathToLocalFile)),
    sink = ltn12.sink.table(response_body)
}
print(response_body[1])

I can help you in creating the mySql table. Take a look at the code below,

  create table imagens (
        imagem_id        tinyint(3)  not null default '0',
        imagem_type      varchar(25) not null default ' ',
        image            LONGBLOB    not null,
        imagem_size      varchar(25) not null default ' ',
        imagem_catg      varchar(25) not null default ' ',
        imagem_name      varchar(50) not null default ' ',
    );

According to PostgreSQL documentation

You can use the bytea format to write binaries:

CREATE TABLE images (imgname text, img bytea);

Creating Lua table

require"luasql.postgres"                       -- carrega a biblioteca postgres
env = assert (luasql.postgres())               -- cria a variável de ambiente
con = assert (env:connect("luasql-test"))      -- conectando ao banco
res = assert (con:execute[[ CREATE TABLE images (imgname text, img bytea); ]])     -- criando tabela binária

Saving data binaries:

res = assert (con:execute[[ INSERT INTO images values ('imagem.jpg', pg_read_file('/caminho/da/imagem.jpg')::bytea) ); ]])