How do Dictionaries Work Within and Between a Class?

I am attempting to understand the connection between the dictionary and the class.

In the code below a dictionary is established in the Assets class. This class manages all of the assets in the project. Other classes can request the images in this Assets class. For example, in the following code the Welcome class is requesting a background image from the Assets class: bg = new Image(Assets.getTexture(”BgWelcome”));

But I don't see how these images are automatically placed in the dictionary. I don't see the connection beyond the fact that they are embedded within the class.

FYI, the example uses Starling API's (e.g. texture)

*Code taken from Hungry Hero Tutorial