Very interesting post by Appsflyer. Let me sum it up for you:
Screenshots - Apps with double portrait screenshots have more opportunities to show their features, they also occupy a larger area than those with a single horizontal screenshot. So, use protrait screenshots, at least for the first 2.
Icons - use color and contrast to your favor - go for the colors that stand out (rather than pastel and other shades), use one elemnt and haveit stand clear against the background.
The rest you can read here: