Obviously, you're on Piwigo.com (so currently this is Piwigo 2.1 but that's not the problem)

The problem is that the photo is not correctly received by Piwigo.

Did you correctly post your form with Content-Type as multipart/form-data ?

oui, multipart/form-data.
voici la requete, telle que vu par Wireshark :

POST /ws.php HTTP/1.1
Content-Length: 7749
Content-Type: multipart/form-data; boundary=E2-8W4Ub3YcIKIATvWI1j5kmizQ4rvefUcw2FmY
Host: mael.piwigo.com
Connection: Keep-Alive
User-Agent: Unit Test
Cookie: pwg_id=754d9a1eda1c40c1781f5e7f5a1d416d
Cookie2: $Version=1

--E2-8W4Ub3YcIKIATvWI1j5kmizQ4rvefUcw2FmY
Content-Disposition: form-data; name="method"
Content-Type: text/plain; charset=US-ASCII
Content-Transfer-Encoding: 8bit

pwg.images.addSimple
--E2-8W4Ub3YcIKIATvWI1j5kmizQ4rvefUcw2FmY
Content-Disposition: form-data; name="category"
Content-Type: text/plain; charset=US-ASCII
Content-Transfer-Encoding: 8bit

98
--E2-8W4Ub3YcIKIATvWI1j5kmizQ4rvefUcw2FmY
Content-Disposition: form-data; name="name"
Content-Type: text/plain; charset=US-ASCII
Content-Transfer-Encoding: 8bit

title1303094948758
--E2-8W4Ub3YcIKIATvWI1j5kmizQ4rvefUcw2FmY
Content-Disposition: form-data; name="file"; filename="piwigo_org.png"
Content-Type: application/octet-stream
Content-Transfer-Encoding: binary

PNG

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--E2-8W4Ub3YcIKIATvWI1j5kmizQ4rvefUcw2FmY--

ah non pardon, tu as ça aussi pour le Content-Type de la requête (je regardais le Content-Type de la variable "method")

Je pense que ça va être compliqué si on compare des dump de versions différentes de wireshark et que ça risque de prendre du temps pour la comparaison...

En espérant que tu es sous Linux, je te propose de faire tourner ce petit script Perl qui fonctionne:

#!/usr/bin/perl

use LWP::UserAgent;

my %conf = (
    base_url => 'http://mael.piwigo.com',
    username => 'mael',
    password => 'HxvDKTM3kGc',
    image_path => '/tmp/piwigo_on_dreamhost.png',
    category => 98
);

my $ua = LWP::UserAgent->new;
$ua->cookie_jar({});

$ua->post(
    $conf{base_url}.'/ws.php',
    {
        method => 'pwg.session.login',
        username => $conf{username},
        password => $conf{password},
    }
);

$ua->post(
    $conf{base_url}.'/ws.php',
    {
        method => 'pwg.images.addSimple',
        image => [$conf{image_path}],
        category => $conf{category},
    },
    'Content_Type' => 'form-data',
);

Il faut changer le mot de passe et le chemin pour l'image bien sûr. Tu sauvegardes ça dans un fichier /tmp/piwigo_addSimple.pl par exemple, et tu le lances avec :

perl /tmp/piwigo_addSimple.pl

A partir du moment où tu arriveras à faire tourner ça sur ta machine, tu auras un point de comparaison fiable pour reproduire la même chose avec Java. Ca te va comme procédure ?

Note : sous Ubuntu, pour avoir le module Perl LWP::UserAgent il faut :

apt-get install libwww-perl