Showing a generic image when no image is chosen
You may want to try this:

{% for image in entry.illustration %}
  {% if image %}
      &lt;a href="{{ entry.url }}" class="flag__img"&gt;&lt;img src="{{ image.getUrl() }}" alt="{{ entry.altText }}" class="profile__image" /&gt;&lt;/a&gt;
  {% else %}
    &lt;p&gt;No image.&lt;/p&gt;
  {% endif %}
{% endfor %}  


(Assuming the altText field is an entry field.) :)