Overwrite field description in an image widget
Using hook_field_widget_form_alter You can do this:

function MYMODULE_field_widget_form_alter(&amp;$element, \Drupal\Core\Form\FormStateInterface $form_state, $context) {
  if ($context['field']['field_name'] == 'field_image_top') {
    // Loop through the element children (there will always be at least one).
    foreach (element_children($element) as $key =&gt; $child) {
      // Add the new process function to the element

      $element[$key]['#process'][] = 'MYMODULE_image_field_widget_process';
    }
  }
}

function MYMODULE_image_field_widget_process($element, \Drupal\Core\Form\FormStateInterface $form_state, $form) {
  // Change the title field label and description

 $element['title']['#description'] = 'SOME NEW DESCRIPTION HERE.';
 return $element;
}