Você pode fazer isso usando o Windows 8 Start Tweaker , role para baixo sob as imagens para encontrar a descrição e as informações e, no lado direito da página, você encontrará o botão / link de download.
Download Windows 8 Start Tweaker zip file. Right-click on the zip file and select Extract all option to get Win8StartTweaker.exe file.
On the first run, you might see the Chinese language. Worry not! Click on the US flag (located at the bottom right) to change the language to English.
Click on the default Start screen background image to select a new image file. For best results, select an image file whose height is double than the screen resolution. For example, if your screen resolution is 1440 x 900, then the height of the image file should be x1800 px.
Click on Metro background image to set a new Metro background color. This color covers the logon screen, Start screen search and some other areas.
Finally click on Apply button to see the change. Click Yes for the prompt to automatically log off and log on your system to apply the changes. Even though the tool lets you restore the original background and color, do make a system restore point so that you can revert to the original Start screen background and Metro color without any issues.
- Fonte: No Windows - Como Para alterar o segundo plano da tela inicial do Windows 8