Custom fields from class-wp-xmlrpc-server.php

Hello,

I’m trying to add some posts by using a custom php file and I got stuck at custom fields.
Everything works fine, I managed to insert title, content, tags, etc, except custom fields.

I see from class-wp-xmlrpc-server.php,
‘custom_fields’ => $this->get_custom_fields($entry[‘ID’])

I want to add at key, a simple word, and on value, an image (eg. wp-content/upload/…..jpg).