<div class="sidebar-header">
              <div class="sh-bg"></div>
              <div class="app-logo">
                 <a href="/">
                   <img src="./assets/img/logo_sm.png" alt="{{ site['name'] }}" class="logo-lg" title="{{ site['name'] }}">
                 </a>
              </div>

Bide şöyle deneyin

<div class="sh-bg"></div>
<div class="app-logo">
<a href="/">
{% if site['logo'] %}
<img src="resimlink" alt="{{ site['name'] }}" class="logo-lg" title="{{ site['name'] }}">
{% else %}
{{ site['name'] }}
{% endif %}
<img src="resimlink" class="logo-sm" alt="">
</a>
</div>