Sure. That's the image with the light-reflecting-wall turned invisible:

That's it with with a visible light-reflecting-wall, but therefore the backdrop isn't visible anymore:

And here's the preview of the settings in the assembly room:

Still very low render settings, but before I continue, I'd love to figure out this light problem.

Thanks a lot for your help!

~Sassy~

Marcelo, I intend to relay on the basics of Populus' image here:
www.renderosity.com/mod/gallery/index.php
What do you think which technique he'd used there? I was hoping for something like in the image this thread started with and which Sandy reworked.

I asked populus about that... lets wait for the answer.

Hi! this image: http://www.renderosity.com/mod/gallery/index.php?image_id=1643367&member
use a spotlight (with softshadows) just outside the window, directed inwards. Indirect lighting turned on and a big white wall on the opposite side of the spot (inside the room, not visible in this camera angle) for those photons to bounce from. A hdri for backdrop but not used for lighting, you can use an normal image for backdrop, work just as well. Hope this helps a little, but these kind of scenes are veeeery tricky to calibrate...took me several hours just to balance the light...

Thanks Populus for the tips.

Thanks a lot for your tips, Populus, I'll try that when I'm at home again.
And thanks for asking him, Marcelo!

~Sassy~