I won't say it's impossible, but it certainly isn't easy. You'd have to alter the exe file of the game itself because I think the people textures are hard-coded.
Now, I have no idea how to do that, because exe files are written in machine language, different from the coding languages like lua and C++, it's the literal language the machine uses internally. I hear that it's very difficult to learn because human language constructs are functionally incompatible with it.

oh ok, thanks