You are posting to very old threads, if not for the list of recent posts on the front page, nobody would even know you had posted. "portraits" is not the same as "Portraits", but I suspect you are in the wrong folder anyways.
There are two folders for Strive, the AppData(saves,mods, and user added pictures) and the application files(.exe, scripts, and game resources). The first folder is the one you need to add portraits.
The in-game way to access this folder is to start the game, start/load a progress, click a slave, click "Customization" button, click "Portraits" button, and click "Open system folder" button. (Each part of the AppData has an access button in a related area.)
The Windows way is to open a file explorer, put "%appdata%" in the address bar, and press "enter" if needed. Find "Strive" and then "portraits".
You can find already assembled packs of pictures here: https://itch.io/board/252654/portarits-packs