In Windows 7, you could change the default Library background image of Windows Media Player 12 (WMP12) by tweaking registry. Unfortunately there is no such provision in case of Windows 8.1 Preview.
I tried to tweak via Registry Editor.
Step By Step Instructions:-
1. Press Win Key and X Key together to launch the Start menu.
2. Click Run.
3. Type in Regedit and press the Enter Key.
4. Click Yes to UAC.
5. Now open the Registry Editor and go to following key:
HKEY_CURRENT_USER\Software\Microsoft\MediaPlayer\Preferences
6. in right-side pane, look for LibraryBackgroundImage DWORD value.
7. Unlike Windows7, You will not get this DWORD value by default.
8. I created a new DWORD (32 bit) value and named it LibraryBackgroundImage.
9. Right click on it and put value data 6.In case of Windows 7 it is 6 by default.
10. I tried to change the values from 0 to 6 to find if the Library Background Image changes-but it remained unchanged.
[The Default background image]
Note:-
[Third party applications are available in the web but I doubt whether they work flawlessly on Windows 8.1 Preview or whether they are compatible.More over playing with the registry is risky and one must back up registry before proceeding.]