The Sprite's Texture property controls which image the Sprite is displaying. This property is of type Texture2D. When a Sprite is created through the SpriteManager's AddSprite method, the Sprite's Texture property gets set inside that method.
The following code explains how Textures are assigned. Files used: Add the following at class scope:
Add the following in Initialize after initializing FlatRedBall:
Add the following in update: