addfieldpath="/modules/mod_social_slider/fields"

<field name="image" type="custom" default="" label="" description="" />

<?php

defined('_JEXEC') or die('Restricted access');

class JFormFieldCustom extends JFormField {

    protected $type = 'Custom';

    protected function getInput() {
        return '<img src="' . JUri::root() . 'templates/your_template/images/image.jpg" alt="" />';
    }

}
?>