As a Broadway usher, I feel somewhat qualified to add my two cents.
Each different theatre has their own policy on whether or not pictures are allowed inside the theatre, even if the show has not yet started. In the time I've been ushering, I've seen a shift in the number that allow it, and now it's almost none. Cameras are getting smaller and more stealth, so it's become theatre policy in most theatres to just put an end to it completely. Also, many theatre designs (as well as show specific designs) are copyrighted and taking pictures is technically illegal, which is another reason pictures are generally frowned upon.
Your best bet is to just ask one of the ushers at the theatre that night what the policy on pictures is. You might get a nice one who will let you hang around in the audience after most people have left and take a quick one. Always better to wait until there aren't many people around, because if other patrons see you do it, you'll set off a horrible chain reaction that the staff of the theatre will then have to deal with.
And if worse comes to worse, I know the New Amsterdam Theatre has a book about it with many wonderful pictures in it. But if you're like me, why pay for a whole book when you can just get a few pics yourself? :lol:
Hope that helped, and if you have any other questions, please feel free to ask.