how to change or modify Preview image on the Vista taskbar?

Question

We have a window like this:

WPF main window (with glass border)
  -> Child window - frame control 
    -> Page 

when user minimize the main window, and to view the preview image on Vista taskbar, the preview image will display main window only, the child window (with page) will missing. so, my question is hot to fix this issue? change or modify the preview image or other ways...