Screenshots are key for converting traffic on your app´s appstore page into downloads. Atomico and Sparkpage are two great tools I use for testing variations of screenshot designs, before uploading them to Apple App Store, Google Play or any other appstore out there. Those tools send traffic to mobile web landing pages, which can be designed like your appstore page. Balance the traffic, compare the CTR and decide which screenshot works best for your app.
How to Run A/B Split Tests for your app´s screenshots