When you start the system, it automatically creates the desktop window. The desktop window is a system-defined window that paints the background of the screen and serves as the base for all windows displayed by all applications.
The desktop window uses a bitmap to paint the background of the screen. The pattern created by the bitmap is called the desktop wallpaper. By default, the desktop window uses the bitmap from a .BMP file specified in the registry as the desktop wallpaper.
The GetDesktopWindow function returns a handle to the desktop window.
A system configuration application, such as a Control Panel applet, changes the desktop wallpaper by using the SystemParametersInfo function with the wAction parameter set to SPI_SETDESKWALLPAPER and the lpvParam parameter specifying a bitmap filename. SystemParametersInfo then loads the bitmap from the specified file, uses the bitmap to paint the background of the screen, and enters the new filename in the registry.