Disabling Wallpaper changes

Is it possible (or will be) to disable wallpaper changes in Vista by group
policy?

I have a problem setting a fix wallpaper to XPs.

I configured User templates\Administrative Templates\Desktop\Active
Desktop\Active desktop wallpaper to enable and set a .jpg file.

When users log on to clients the wallpaper is configured exactly but
transparency goes off.

I controlled every part of Registry, Use drop shadow (checked), lock desktop
items(unchecked).

Active desktop overrides transparency.

So I want to wait Vista to apply this policy. Did you test a policy like
this before on Vista? Was it behaving like XP?

Thanks for your help...

Cuneyt