How do you change avatar preview image

using the custom avatar uploader, it gives you an option to upload an avatar with a custom image but it doesn't really elaborate on how to do it and I couldn't get it to work, could anyone please explain how it works

I reccomend checking out this thread
https://steamcommunity.com/app/438100/discussions/1/2828702373000761142/

or watching this video
Custom Social Avatar Profile Picture on VRChat - Tutorial - Lesson – 08:08
— Comfy Chat

I hope this is helpful !