Example of implementation of a hook validate #drupal

gistfile1.php

<?php 

/**
 * Implements hook_node_validate().
 *
 * Makes image field required.
 */
function mymodule_node_validate($node, $form, &$form_state) {
  if ($node->type == 'carrousel_item') {
    if (! isset($form_state['values']['field_summary_image'][LANGUAGE_NONE]['0']['entity']->field_media_summary_image[LANGUAGE_NONE][0]['fid']) ||
      $form_state['values']['field_summary_image'][LANGUAGE_NONE]['0']['entity']->field_media_summary_image[LANGUAGE_NONE][0]['fid'] == 0) {
      form_set_error('field_summary_image', t('Image field is required'));
    }
  }
}