web/_includes/img-url.html

{%- comment -%}
  Generate image final URL based on `site.img_cdn`, `page.img_path`

  Arguments:
    src - basic image path, required
    img_path - relative path of image, optional

  Return:
    image URL
{%- endcomment -%}

{% assign url = include.src %}

{%- if url -%}
  {% unless url contains ':' %}
    {%- comment -%} CND URL {%- endcomment -%}
    {% assign prefix = site.img_cdn | default: '' | relative_url %}

    {%- comment -%} Add page image path prefix {%- endcomment -%}
    {% assign url = include.img_path | default: '' | append: '/' | append: url %}

    {% assign url = prefix
      | append: '/'
      | append: url
      | replace: '///', '/'
      | replace: '//', '/'
      | replace: ':', ':/'
    %}
  {% endunless %}
{%- endif -%}

{{- url -}}