Which patents should I study to avoid lawsuits later?

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.