gnrocks

You are right, i should have mentionned my host !
It is iWeb.

I finally had an answer from my webhost.

1) create a php.ini that will contain following :

<?php
memory_limit = 64M
?>

2) upload it to the public_html folder

3) open the .htaccess file located in the public_html folder and add the following :
suPHP_ConfigPath /home/domaine_username/public_html/

Replace domain_username by your username for the cpanel.

Your .htaccess may look like (the code between <ifmodule>…</ifmodule> can vary ), the important is to add the sentence before #END WordPress:

# BEGIN WordPress

<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]

</IfModule>
suPHP_ConfigPath /home/domaine_username/public_html/

# END WordPress

And that’s it!
Enjoy!