Today you can set an animated picture or video as a background in Windows 8 using Windows 8 Start Screen Customizer this is a free tool that let’s you set custom image at the Metro UI you can also put animated pictures and videos as a background.
1. Download and install Windows 8 Start Screen Customizer a very basic installer, then run the program on your windows 8 or 8.1 machine.
2. When it’s opened, select your custom media file by clicking on Load Picture under the image preview.
3. When the windows explorer appears, Brows for the the image you want to use.
4. When you select your image it will appear in the preview windows where you can crop it and adjust other image settings before you put it as a start menu background . when you’re done you can click on Apply & Save.
5. After you click on Apply it will automatically redirect you to your metro UI start screen to view the new image there.
NOTE:If you’re trying to put a video as a background and it’s not working the problem can be the video format, if it is a rare format the software can’t recgonize it.