How are you going to handle gathering model releases for the individuals in the photos? For example, iStockPhoto has detailed requirements on which photos require a model release (http://www.istockphoto.com/tutorial_9.0_modelrelease.php).
That's a good point. I'm not sure if this will fly, but as I understand it, getting a model release is the photographers responsibility. I will simply require them to "agree" to a statement that declares they have the permission of all people in the photo.
While you can probably indemnify yourself against legal responsibility of a photographer not getting a model release, based on your target photographers (teens and twenty somethings taking photos with camera phones i.e. in a hurry), this is something that could quickly kill your reputation if someone uses one of your images and gets sued because the photographer never got a model release. One suggestion on this would be to have something in your mobile app that would allow the user to collect the model release at the time of capture. I'm sure there are a lot of legal issues with what would be considered an appropriate signature.
One of the reasons people feel comfortable using crowd sourced stock photo sites like ShutterStock and iStockPhoto is that they are willing to take up to a certain $ amount of the legal responsibility you get sued for using an image (as long as you use it within their guidelines). If your target buyers are news outlets (who are allowed to use images more liberally than for commercial use) this might not be a huge issue, but I'd be kind of wary about using a stock photo site that basically took the stance that I had to counter sue the photographer. Not a idea killer, but it's worth looking at what the established players do to see if it makes sense from your business perspective.