How do I check if an image field is empty in a template file?
Use the method isEmpty() on the image field:

  {% if node.field_image.isempty %}
    &lt;p&gt;Field "field_image" is empty.&lt;/p&gt;
  {% endif %}


Although in this case you might need to check the user entity of the node author:

  {% if node.uid.entity.user_picture.isempty %}
    &lt;p&gt;Picture of the node author is empty.&lt;/p&gt;
  {% endif %}