fix djen of django cover

Author: revolunet

issues.json

@@ -402,7 +402,7 @@
       "level":"Intermediate",
       "info":"Djen of Django is a book consisting of a series of small Django projects based on small real-world examples. For instance, building a Pastebin, a Blog or a Project Management Application. Djen of Django focuses on teaching the reader Django best practices through the use of real-world examples.",
       "url":"http://agiliq.com/books/djenofdjango/",
-      "cover": "img/djenofdjango.png"
+      "cover": "img/djenofdjango.jpg"
     }
 
 ]}