Use a set_rules callback in a libraries

You can extend CI_Form_validation and define new callbacks there.

Some notes about your implementation of MY_Form_validation.

You do not need to add the property $CI. The parent class CI_Form_validation already has that property and it is accessible in the extending class.
You don't need MY_Form_validation::__construct because it does not do anything besides call parent:: __construct. PHP will call the parent constructor without your help.
In the new callback you don't have to find the instance of form_validation because you're in the instance you're looking for.

All that said, your class should look like this.

PHP Code:

class MY_Form_validation extends CI_Form_validation
{
    function require_upload()
    {
        $this->set_message('require_upload', "Vous devez uploader une image");
        return false;
    }
} 


As you stated, it would be used like this

PHP Code:

if($config_rules['images'] == true){ $this->form_validation->set_rules('images','images','require_upload');} 


Not sure why you have the callback always returning false. Maybe you're not done writing it yet?