updates screenshot data [ci skip]

guides/source/getting_started.md

@@ -187,7 +187,7 @@ This will fire up WEBrick, a web server distributed with Ruby by default. To see
 your application in action, open a browser window and navigate to
 <http://localhost:3000>. You should see the Rails default information page:
 
-![Welcome Aboard screenshot](images/getting_started/rails_welcome.png)
+![Welcome aboard screenshot](images/getting_started/rails_welcome.jpg)
 
 TIP: To stop the web server, hit Ctrl+C in the terminal window where it's
 running. To verify the server has stopped you should see your command prompt