I have a client that wants to build an iPhone/Android game. The game is a photo scavenger hunt. The basic premise of this game is very similar to Scavenger Hunt with Friends. You can see it's info here: [url removed, login to view] The main difference between that game and this one is the use of a coin system for in game purchases, the ability to buy and add masks to conceal the identity of people in the photos, a challenge system to prove that the photos were taken and not just uploaded. There is also a wagering system for photo challenges. This is all diagrammed in the enclosed graphics.
There is a lot of information here and I am sure that you will have questions. Please look over the information and contact me and I'll provide whatever info that you need to make an accurate estimate of the time and cost.