How can I add external-assets(C#)

Hi guys here's my question How do I code unity to add a model after it has been built.let me explain a little. Let's say I have a game which takes 2d images and places them in a specific place in the UI as avatar. I want to be able to read the files from a folder outside the game after it his been built.

As example lets say after I build the game I get the game .exe file and a 'data' folder. I want to add another folder called 'avatar pics' in which players can save their custom pics and the game will import them and put them in a selection screen when it runs. Any help will be much appreciated.