Thanks for your advice. There are couple competitors on a market right now ( launchkit.com, storeshots.net etc. ) and they have richer functionality than my service. So i'm not sure why people would pay for my app rather than for theirs.
Half a year ago i've started http://www.appstoreoptimization.io/ - screenshot generator for app/play stores. For now there are no paid plans as i want to deliver as much useful functionality as possible before charging people