First of all make a backup copy of %windir%system32wmploc.dll file because we are going to edit this file which store all images of WMP,make its a backup copy and store’s it at a safe place so that you can recover it in future if anything went wrong.
Ok now open %windir%system32wmploc.dll open in Resource Tuner and expand 257 key.
Now here you need to know that LIBRARY_BACKGROUND2.PNG’s value in Registry is 1 and LIBRARY_BACKGROUND7.PNG’s value in Registry is 6 and so on.
Now select any image (according to the value which you had set in registry,If you dont have made any changes and using WMP as default then go to LIBRARY_BACKGROUND7.PNG).
Now right click on it and select Edit Resource and then on the new dialog box you will see your default image at top, now click on the small folder at the center left just below default image.
Now it will open another dialog box locate your image and open it and click ok It will ask to proceed click OK.
Now at the end save it and close Resource Tuner and check it.
P.S.:- You may need to take ownership of %windir%system32wmploc.dll so take a look at HERE