Try to give a different style to directories.

sigal/themes/colorbox/static/css/style.css

@@ -40,6 +40,20 @@ header h1 a {
   box-shadow: 0 0 5px #818181;
 }
 
+.thumbnails {
+  position: relative;
+}
+.album_thumb_prev {
+  position: absolute;
+  transform: rotate(-5deg);
+  z-index: -1;
+}
+.album_thumb_next {
+  position: absolute;
+  transform: rotate(5deg);
+  z-index: -1;
+}
+
 .album_title {
   display: block;
 }

sigal/themes/colorbox/templates/index.html

@@ -25,9 +25,14 @@
       <div class="{{ column_size_t }} columns thumbnail
                   {% if loop.index % nb_columns == 1 %}alpha{% endif%}
                   {% if loop.index % nb_columns == 0 %}omega{% endif%}">
-        <a href="{{ alb.url }}">
+        <a href="{{ alb.url }}" class="album_thumb_container">
+          <img src="{{ alb.thumbnail }}" class="album_thumb_prev"
+              alt="{{ alb.title }}" title="{{ alb.title }}" />
+          <img src="{{ alb.thumbnail }}" class="album_thumb_next"
+              alt="{{ alb.title }}" title="{{ alb.title }}" />
           <img src="{{ alb.thumbnail }}" class="album_thumb"
-              alt="{{ alb.title }}" title="{{ alb.title }}" /></a>
+              alt="{{ alb.title }}" title="{{ alb.title }}" />
+        </a>
         <span class="album_title">{{ alb.title }}</span>
       </div>
     {% if loop.last or loop.index % nb_columns == 0 %}