I structured the code in a similar way to some of my personal projects. Sometimes the scope of what is needed in an app can change dramatically. Even though URLSession would have worked fine in this instance, I used Alamofire because it provides a simpler interface to more complex requests. Alamofire Image provides an easy caching mechanism and a simple API to cancel image requests both of these features are in this app.
Ideas
Right now the loading graphic for gravatars is an activity indicator. I was thinking for Wag it would be cool to implement a wagging tail as a loading indicator