{% if page.bg %}
  <aside class="sidebar" role="note" style="background-image: url({{ site.url }}{{ site.upload_dir }}/{{ page.bg }})">
{% else %}
  <aside class="sidebar" role="note" style="background-image: url({{ site.url }}{{ site.upload_dir }}/default-post-bg.jpg)">
{% endif %}

  <div class="cover">

    <div class="cover-text">
      <div class="heading">

        {% if page.tags %}
          {% for tag in page.tags %}
            <a href="/posts/#{{ tag | downcase }}">{{ tag | downcase }}</a>
          {% endfor %}
        {% else %}
          {{ page.title }}
        {% endif %}

      </div>

      <p>
        {% if page.summary %}
          {{ page.summary }}
        {% endif %}
      </p>

    </div>

  </div>

  <div id="switcher"></div>
</aside>

{% if page.bg %}
  <aside class="sidebar" role="note" style="background-image: url({{ site.url }}{{ site.upload_dir }}/{{ page.bg }})">
{% else %}
  <aside class="sidebar" role="note" style="background-image: url({{ site.url }}{{ site.upload_dir }}/default-post-bg.svg)">
{% endif %}

  <div class="cover">

    <div class="cover-text">
      <div class="heading">

        {% if page.tags %}
          {% for tag in page.tags %}
            <a href="/posts/#{{ tag | downcase }}">{{ tag | downcase }}</a>
          {% endfor %}
        {% else %}
          {{ page.title }}
        {% endif %}

      </div>

      <p>
        {% if page.summary %}
          {{ page.summary }}
        {% endif %}
      </p>

    </div>

  </div>

  <div id="switcher"></div>
</aside>

Update default side bar image
Update default side bar image
