One of the new features of macOS Mojave is the Dynamic Desktop, which, when enabled, changes the image on your Mac display depending on the time of day.
The Dynamic Desktop wallpaper requires you to enable Location Services as it requires access to your location. To do this, open System Preferences ⟶ Security & Privacy ⟶ select Privacy tab and then check the box for Enable Location Services.
Next go to System Preferences > Desktop and Screen Savers.
Click the Desktop tab and choose Dynamic wallpaper from the available options.
To disable it (something you may wish to do on a laptop that’s running on its battery, as a changing screensaver eats up more battery life than a static one), click on Apple menu ⟶ System Preferences ⟶ Desktop and Screen Savers. Set another wallpaper.