They are themes for your desktop. If I go to my "Region and Language" settings and set my location to Canada it adds a Canada theme to the ones available when you go to the personalization screen. As part of that Aero theme I then get the wallpapers that are in the MCT-CA/Canada folder. MCT-AU is Australia, MCT-GB is United Kingdom. MCT-US is United States and MCT-ZA is I believe South Africa. You can also download and install them by clicking the "get more themes online" on the Personalization screen where you pick your Aero theme. The easiest way to install them though is to go to each one of the "Theme" folders and double click the .theme file that is in it. You'll hear a confirmation beep and the Personalization screen will open automatically with that theme highlighted.