Question I need help to create my portrait mod :,)

[lendvx]

Hi, I'm an artist and i would like to create HD portraits, but I'm struggling with modding because I need to make the portrait fit in the game and not pixelated. I just want to do that for my game, but i would love to post a portrait mod one day :) Do I really have to use SMAPI and CP ? Also I found that on a forum, but i dont know how to make it work :

I hope someone can help !