Use .specific to return this page in its most specific subclassed form.

opentech/templates/base.html

@@ -70,8 +70,8 @@
         {% block header %}
             {% if page.header_image %}
                 {% image page.header_image fill-2560x410 as header_image %}
-            {% elif page.get_parent.header_image %}
-                {% image page.get_parent.header_image fill-2560x410 as header_image %}
+            {% elif page.get_parent.specific.header_image %}
+                {% image page.get_parent.specific.header_image fill-2560x410 as header_image %}
             {% endif %}
             <header class="header header--standard {% if page.header_image %}header--has-bg-image{% endif %} {% block header_modifier %}{% endblock %}" {% if header_image %}style="background-image:url('{{ header_image.url }}')"{% endif %}>