Change Windows 8 Lockscreen image with wpf/service only works once

I'm trying to Change the lockscreenimage with a service. But it only works once. If I try it a second time (i attached a timestamp text to the Image with drawtext), the new Image is shown correctly in the Settings pane inside the lockscreen Settings Dialog but is not shown on the lockscreen itself.

Are there some known restrictions?

I followed These steps here: http://www.codeproject.com/Questions/709599/Csharp-code-to-change-lockscreen-and-assign-file-t
and here: http://stackoverflow.com/questions/20103561/wpf-application-incorporating-winapi-win8-components

Then I've Access to the needed assamblies and able to call await LockScreen.SetImageFileAsync(imageFile);

As said... works fine the first time, but Fails the second.
I tried to run the code as a service, as a console applicationd and as aregular wpf application.