If I remember correctly first you create an icon with dimensions 1024x1024 and then you create a folder named "Special" in folder "Files" and and you save it in there with the name icon-store-1024.png.
Edit - I remember correctly :
This tutorial is relevant for B4i v4.40+. App icons iOS apps require a wide range of icon sizes. Starting from 20x20 and up to 1024x1024. B4i will create all icons automatically based on the large icon (icon-store-1024.png). Start by putting your own image in \Files\Special. The file name...