Do not assume images would be placed in /assets/images/

In order to add an image to a post, just use the following syntax:

{% include image image_path="/full/path/to/MyPostImage.png" caption="The optional image caption" %}

or

{% include image image_path="https://example.com/images/MyPostImage.png" caption="The optional image caption" alt="The optional alt text" %}

_includes/image

@@ -3,7 +3,7 @@
     {% if include.image_path contains "://" %}
       "{{ include.image_path }}"
     {% else %}
-      "{{ include.image_path | prepend: "/assets/images/" | absolute_url }}"
+      "{{ include.image_path | absolute_url }}"
     {% endif %}
     alt="{% if include.alt %}{{ include.alt }}{% endif %}">
   {% if include.caption %}