Facebook is not an optimal platform given your desired narrow target audience, it will get lost in the noise. You need to target the proper end user... you are a pic based service as such you should be advertizing on instagram. These are the people that probably want to know which is the best next pic to post to their followers. The hook shouldn't be to compare shopping etc the immediacy of confirmation as a pre-screener prior to social media uploads so that the end user can put their best shots out to the world would be the interesting proposition. The audience is already photography focused and the psychology of avid instagrammers, their inherent insecuries should be taken advantage of. People never take just one picture, they take multiple shots, different angles etc You would do well to position yourself as the app with the answer. People are also impressionable and want input from trusted friends, they want the burden of choice to be aleviated, to be done for them even. They do not know this, but they do it, I see it all the time, human nature is predictable, take advantage of it and leverage the ads to maximise this (tact and subtlety should be used, people dont like to be told they are insecure, the product should feel as though it is answering their inner concerns without them realizing it, ergo, and indispensable extension of the social platorm powered by the input of their most trusted friends to etade doubt and augment confidence that the post will be the best one possible). The app should leverage instagram, insofar as the winning pic being immediatly uploadable to their profile. This synergy should not be overlooked. Otherwise, I hope you also have an active instagram profile and are leveraging tags etc to get your app in front of eyeballs for free. Another avenue to explore for ads would potentially be snapchat though I am much less familiar with them with respects to advertizing.