I am not a Marketing Guru but, I would suggest you to have referral system and Free plan and restrict usage of your core service. In your case, I would restrict

I am not a Marketing Guru but, I would suggest you to have referral system and Free plan and restrict usage of your core service. In your case, I would restrict the number of photos user can upload every month. This way users will get change to use all the features of your app and if it works for him, he can upgrade his account. Word of mouth is another best marketing channel. Also, you can sponsor bloggers and in return they will advertise your product. Eg. TutsPlus.