add default image to learning object

app/assets/stylesheets/application/learning_objects.scss

@@ -13,8 +13,8 @@
   }
 
   .learning-object-thumbnail {
-    width: 250px;
-    height: 150px;
+    min-width: 250px;
+    height: 141px;
     position: relative;
 
     .learning-object-actions {
@@ -23,5 +23,4 @@
       right: 5px;
     }
   }
-
-}
\ No newline at end of file
+}

app/views/learning_objects/_learning_object_horizontal.erb

+<% link = learning_object_path(id: learning_object.id) %>
 <div class="learning-object-horizontal">
   <div class="media">
     <div class="media-left media-middle">
       <div class="learning-object-thumbnail">
-        <a href="#">
-          <img src="http://dummyimage.com/250x150/363336/fff" alt='dummy image'>
+        <a href="<%= link %>">
+          <% if learning_object.thumbnail == "thumbnai" %>
+            <%= image_tag('learning-object-preview.png') %>
+          <% else %>
+            <img src="<%= learning_object.thumbnail %>" alt="Prévia do objeto">
+          <% end %>
+          <%= render 'learning_objects/actions_buttons' %>
         </a>
-
-        <%= render 'learning_objects/actions_buttons' %>
       </div>
     </div>
     <div class="media-body">