I think they pretty much redecorate the set a lot because I never recognized that picture either and they already had some shots on the stairs. Or maybe I just didn't really pay attention.

You can actually see the pic quite better right at the beginning, when Castle finds his daughter jummping in the hall. Martha comes down the stairs and the pic is hanging right beside her on the wall.

Well I think I have found it:  http://upload.wikimedia.org/wi….._metro.jpg

On wikipedia it´s flipped, but if you would flipp it back it turns out to be just the pic Castle has. I think you can see that with the help of the lamp which is hanging in the front. On Castles pic is a red shade in the beginning which I guess comes from Martha, while she is discending the stairs.

On wikipedia they write that this pic shows the Metrostation Cité and if you enter that in google pic search you can find that: http://images.google.de/images…..amp;tab=wi … so I guess wiki is right, it is the Metrostation Cité.

I have not so far found out, why this pic might be famous for something… I have neither found a photographer nor a Museum it might be in…So I can´t really say anything about it.

It would be interesting to know why Castle actually chose this pic to hang in his apartment…