Assuming that this is the game in question:

https://steamdb.info/app/640820/

It looks like it has a Linux-native build, which is probably what you are using. Anything under pfx/drive_c is used by Proton, Valve's WINE build used for running Windows binaries. You can force use of a game's Windows binary under Proton rather than the Linux-native binary, in which case all of the Windows file paths for game-modifiable stuff will be in a game-specific directory under:

~/.steam/steam/steamapps/compatdata/<Steam AppID>/pfx/drive_c/<Windows path>

There can also be game data files in ~/.steam/steam/steamapps/common/<game name>. I believe that that's just intended for stuff that Steam writes, not stuff that the game might modify.

If the above-linked game is the one you're talking about, then its AppID is "640820".

The path below that will be the same as for the Windows version of the game; just won't have a "C:\\" prefix, but the ".../drive_c" prefix.

But if you're using the Linux-native build, which I'd guess is most-likely the case, then a game technically could be storing it wherever. It'll most-likely be under ~/.local (or ~/.config, though that's not really intended for binary game data) somewhere. Can do:

$ find ~/.local | grep Portraits

And it'll list paths containing "Portraits".

kagis

https://old.reddit.com/r/Pathfinder_Kingmaker/comments/mki0sa/portraits_on_linux/

This guy has:

~/.config/unity3d/Owlcat Games/Pathfinder Kingmaker/Portraits/