Choose-Image-Style-Drupal-8.twig

{% set image_file_path = content.field_teaser_image[0]['#media'].field_media_image.entity.uri.value %}
{% set image_file_alt = content.field_teaser_image[0]['#media'].field_media_image.0.alt %}

<div class="teaser__image">
  {% if node.field_teaser_image_style.value == 'portrait' %}
    <a href="{{ url }}">
      <img src="{{ image_file_path | image_style('portrait_teaser') }}" alt="{{ image_file_alt }}">
    </a>
  {% endif %}

  {% if node.field_teaser_image_style.value == 'landscape' %}
    <a href="{{ url }}">
      <img src="{{ image_file_path | image_style('landscape_teaser') }}" alt="{{ image_file_alt }}">
    </a>
  {% endif %}
</div>

For my own reference, background to this snippet here https://www.annertech.com/blog/allow-editors-choose-image-style-drupal-image-fields