Registry

HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Policies\ActiveDesktop\
  • NoChangingWallPaper ( 0 = False / 1 = True [nedovoli zmenu pozadi] )
HKEY_CURRENT_USER\Control Panel\Personalization\Desktop Slideshow\
  • Interval (the slideshow duration in miliseconds)
  • Shuffle ( 0 = False [not checked] / 1 = True [checked] )

Odkazy 

http://www.howtogeek.com/howto/16929/prevent-users-from-changing-screen-saver-and-wallpaper-in-windows-7/

http://www.pctools.com/guides/registry/detail/1097/

Slideshow.ini 

You can find wallpaper location and wallpaper fit in HKEY_CURRENT_USER\Control Panel\Desktop.
The information about slideshow is not in Windows registry. You have to open path:

%USERPROFILE%\AppData\Roaming\Microsoft\Windows\Themes\

and make sure, that hidden files are visible (you can change that in folder options).
You can see slideshow.ini. There are tags for root slideshow image and slideshows. I don't know how to decode it. Have fun :)