First mover advantage, but it'll only take off if you work really hard in the first month or so to get the word out and become a 'household name'.
That said, yeah it's likely there will be competitors springing up left and right if it becomes popular (recent example is this "unpacking" game, the app store was flooded with clones within a month). For apps like this, your best hope is probably that a bigger party like Instagram knocks on your door and offers to buy you out to integrate the feature into their own apps. It's cynical, but this is where we find ourselves; I don't believe single feature apps have much of a future.
A photo manipulation app (generally speaking) is great, but unless you add a social network to it like Instagram did it's not going to go far.
If he had wanted to make serious money as an app developer, he wouldn’t have quit his day job.