Creating a fallback field to load conditionally
If field_secondary is also an image field, then you can check if it is empty in your Twig template:
For example, in node--article.html.twig:
{% if content.field_secondary is empty %}
  {{ content.field_image }}
{% else %}
  {{ content.field_secondary }}
  {{ content.field_image }}
{% endif %}

{{ content.without('field_image', 'field_secondary') }}

(Your actual template will have more HTML in it.)