It's not hard to see that Pierre ingame sprite looks nothing like his portrait at all. This is my take on how he should look like with his current vanilla portrait: Hair is pretty much redone with color similar to portrait, also changed the way his glasses look so it looks less ridiculous (the glasses "structure" is similar to the way Harvey's glasses look in Harvey Refined by Chrysanthe, which is an inspiration for this mod!).
I know this doesn't look perfect but I've tried my best.

As of update 1.5, I've added an alternative version. This version is simply Pierre vanilla sprite but I only changed his hair color to match with his portrait, this is for those who prefer the way he looks in vanilla version (those ridiculous glasses is kinda cute anyway).

Note:
This mod doesn't change his portrait, only his sprite so that he matches his vanilla portrait.

How to install:
Simply put the content in your Stardew Valley folder.

Permission:
You are free to do whatever you want with my mods as long as you credit me.