Optimize getting started guide images, and use png instead of jpg

We noticed the rails gem increased in size between the beta and rc
versions, and the reason is that some guides images were updated but not
properly optimized, and one image changed from png to jpg, way bigger.

Converting the jpg image back to png, and using ImageOptim to optimize
all the images for web view, improves the situation [ci skip].

---

In current master/4-1-stable:

    508K  guides/assets/images/getting_started/

In v4.1.0.beta1:

    268K  guides/assets/images/getting_started/

In 4-0-stable

    328K  guides/assets/images/getting_started/

After the patch:

    260K  guides/assets/images/getting_started/

guides/source/getting_started.md

@@ -206,7 +206,7 @@ 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.jpg)
+![Welcome aboard screenshot](images/getting_started/rails_welcome.png)
 
 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