Controllers are mapped to actions (one route perform one action). You can call a controller by calling his route.
Because getting images, and saving them are part of the same action (ie: same route), I recommend you to move that code to a library or helper (or why not a model, because you are retrieving and saving data)
Anyway, I don't get the reason why you are doing that in controllers. Controllers usually perform operations between models and the views. Fetching data from a webservice belong more to models than to controllers. In fact, I really recommend you to move that code to a library to be reusable later. Your controller must only have the responsibility of calling the library's read method to read the image, then saving it locally using a model.