If you’re making your own app icons, you should create PNG files and stick to the following color spaces: sRGB (color) or Gray Gamma 2.2 (grayscale), according to Apple. How to change an app ...

If you want your photos in PNG, you need to convert them from HEIC or JPG using the Shortcuts app. 1. If it's not already on your iPhone, install the Shortcuts app from the App Store.

The original app icon will still be there, but you can hide it from the home screen to avoid confusion. Just hold your finger on the screen until the icons start to shake and click the minus sign.