Here is a guide on how to design screenshots for publishing apps on the App Store.
We need four groups of screenshots which contain three sizes for iPhone devices and one size for iPad devices.
iPhone Screenshots
The details of each iPhone size and its examples are viewed in the image below:
5.5 iPhone
6.5 iPhone
6.7 iPhone
Note that the device used in iPhone screenshots is a real iPhone.
iPad Screenshots
The details of iPad size and its examples are viewed in the image below:
iPad
Note that the device used in iPad screenshots is a real iPad.