Fix image names in introductory popup

This will make sure browsers don't display old, cached images to clients.

app/views/projects/index/_introductory_popup.html.erb

@@ -8,16 +8,16 @@
       </div>
       <div class="modal-body">
         <p><%= t('introductory.sentence_one_html') %></p>
-        <%= image_tag 'introductory_popup/image1.png', class: 'img-responsive center-block' %>
+        <%= image_tag 'introductory_popup/image1a.png', class: 'img-responsive center-block' %>
         <p><%=t 'introductory.sentence_two_html' %></p>
         <p><%=t 'introductory.sentence_three_html' %></p>
         <p><%=t 'introductory.sentence_four_html' %></p>
-        <%= image_tag 'introductory_popup/image2.png', class: 'img-responsive center-block' %>
+        <%= image_tag 'introductory_popup/image2a.png', class: 'img-responsive center-block' %>
         <p><%=t 'introductory.sentence_five_html' %></p>
         <p><%=t 'introductory.sentence_six_html' %></p>
-        <%= image_tag 'introductory_popup/image3.png', class: 'img-responsive center-block' %>
+        <%= image_tag 'introductory_popup/image3a.png', class: 'img-responsive center-block' %>
         <p><%=t 'introductory.sentence_seven_html' %></p>
-        <%= image_tag 'introductory_popup/image4.png', class: 'img-responsive center-block' %>
+        <%= image_tag 'introductory_popup/image4a.png', class: 'img-responsive center-block' %>
       </div>
       <div class="modal-footer">
         <button type="button" class="btn btn-default" data-dismiss="modal"><%=t 'introductory.confim' %></button>