As a photographer I can tell that retouching a very detailed panel with photoshop would be a real pain. If you really want to do it properly (and the panel is not too big) you have to use diffused studio lights to light the window. This has the advantage that you can light it from inside and take the photo from outside. Or hanging a white sheet in front might help. If the panel is really huge, try doing it at night from outside with the church lights on and using a time exposure & tripod. Hope this helps.