Which patents should I study to avoid lawsuits later?

I would also add that the FTO search will not give the certainty of avoiding being sued in the future, as even the best done search cannot ensure that all relevant patents have been discovered and analyzed.

What the FTO will give you is a realistic patent landscape and a realistic analysis of where the risks lie or not.

You would probably need to consult a patent attorney who has expertise in the app field. What you are essentially asking for is a freedom to operate search - your objective is to uncover any patents which you may potentially infringe and therefore want to stay away from. The FTO search is designed to uncover those patents whose claims could potentially be problematic for your social photo app. If you do not run afoul of any claims of any patents, then hopefully you should have the freedom to operate without infringing any patents if the FTO search was in fact conducted thoroughly and accurately.