A menorah that is lit for decoration does not get any blessing, as you are not performing any mitzvah. An analogy (somewhat) is that in many shuls, a menorah is lit during the day, but, as there is no mitzvah of lighting at day time, no bracha is made.
One solution, perhaps, is that if you want an extra family member to light his / her menorah at night, then that person would make a bracha on the lighting, and you end up having an extra lit menorah around.
There may possibly be a problem of benefitting from the lit menorah candles as you are using it in a picture. (Ask a rav about this, because a picture is viewed much later, so I don't know if this is called "benefit".) At any rate, to solve the doubt, wait the minimal 1/2 hour or until the candles are out. After that, light as many menorot as you wish without a bracha and take all your pictures. Feel free to upload one, here.