Test its existence before using thumbnail_src

Thanks @pointhi !

searx/templates/courgette/result_templates/images.html

@@ -1,6 +1,6 @@
 <div class="image_result">
     <p>
-        <a href="{{ result.img_src }}"><img src="{{ image_proxify(result.thumbnail_src) }}" title="{{ result.title|striptags }}" alt="{{ result.title|striptags }}"/></a>
+        <a href="{{ result.img_src }}"><img src="{% if result.thumbnail_src %}{{ image_proxify(result.thumbnail_src) }}{% else %}{{ image_proxify(result.img_src) }}{% endif %}" title="{{ result.title|striptags }}" alt="{{ result.title|striptags }}"/></a>
         <span class="url"><a href="{{ result.url }}" class="small_font">{{ _('original context') }}</a></span>
     </p>
 </div>

searx/templates/default/result_templates/images.html

@@ -1,6 +1,6 @@
 <div class="image_result">
     <p>
-        <a href="{{ result.img_src }}"><img src="{{ image_proxify(result.thumbnail_src) }}" title="{{ result.title|striptags }}" alt="{{ result.title|striptags }}" /></a>
+        <a href="{{ result.img_src }}"><img src="{% if result.thumbnail_src %}{{ image_proxify(result.thumbnail_src) }}{% else %}{{ image_proxify(result.img_src) }}{% endif %}" title="{{ result.title|striptags }}" alt="{{ result.title|striptags }}" /></a>
         <span class="url"><a href="{{ result.url }}" class="small_font">{{ _('original context') }}</a></span>
     </p>
 </div>

searx/templates/oscar/result_templates/images.html

@@ -1,7 +1,7 @@
 {% from 'oscar/macros.html' import draw_favicon %}
 
 <a href="{{ result.img_src }}" data-toggle="modal" data-target="#modal-{{ index }}">
-    <img src="{{ image_proxify(result.thumbnail_src) }}" alt="{{ result.title|striptags }}" title="{{ result.title|striptags }}" class="img-thumbnail">
+    <img src="{% if result.thumbnail_src %}{{ image_proxify(result.thumbnail_src) }}{% else %}{{ image_proxify(result.img_src) }}{% endif %}" alt="{{ result.title|striptags }}" title="{{ result.title|striptags }}" class="img-thumbnail">
 </a>
 
 <div class="modal fade" id="modal-{{ index }}" tabindex="-1" role="dialog" aria-hidden="true">
@@ -12,7 +12,7 @@
                 <h4 class="modal-title">{% if result.engine~".png" in favicons %}{{ draw_favicon(result.engine) }} {% endif %}{{ result.title|striptags }}</h4>
             </div>
             <div class="modal-body">
-                <img class="img-responsive center-block" src="{{ image_proxify(result.thumbnail_src) }}" alt="{{ result.title|striptags }}">
+                <img class="img-responsive center-block" src="{% if result.thumbnail_src %}{{ image_proxify(result.thumbnail_src) }}{% else %}{{ image_proxify(result.img_src) }}{% endif %}" alt="{{ result.title|striptags }}">
                 {% if result.content %}<p class="result-content">{{ result.content|safe }}</p>{% endif %}
             </div>
             <div class="modal-footer">