How I can intercept an image field upload and validate it?
It looks like there a couple of very small bugs in that code that would stop it working properly, try this:

$lang = $form['field_all_images']['#language'];
foreach (element_children($form['field_all_images'][$lang]) as $key) {
  $form['field_all_images'][$lang][$key]['#upload_validators'][] = 'check_image_correct';
}