By default, it will be synced across all your devices. If your account is a Microsoft Account, the image you set will be uploaded to Microsoft's servers and used in all their cloud services like OneDrive, Office 365 and so on. Instead of the default image, you can use your favorite wallpaper or your real photo. It is also visible in the Start menu as a tiny round thumbnail. The user picture will be visible every time you sign in with your Windows 10 account.
