Bildstil (image style) im Template überschreiben - Drupal 8

field--field-paragraph-gallery-photos.html.twig

{# main image #}
<ul{{ attributes.addClass(classes) }}>
  {% for item in items %}
    <li{{ item.attributes.addClass('field__item') }}>
      {{ item.content }}
    </li>
  {% endfor %}
</ul>

{# thumbnails #}
<ul class="gallery__navigation">
  {% for item in items %}
    <li class="gallery__navigation-item">
      {% set content = item.content %}
      {% set content = content|merge({'#image_style': 'my_thumbnails_style'}) %}
      {% set item = item|merge({'content': content}) %}

      {{ item.content }}
    </li>
  {% endfor %}
</ul>