As in saving the .png with transparency layer intact ? I guess.

It's also possible to remove background from pictures, game will put background and lightning over portrait itself, if i remember correctly

Thanks mate

Dont know what the files exactly are, but they are necessary for custom portraits to work;
They are generated there by the game;

Maybe you can add more custom portraits by copy pasting the .xnb files and adding different name (like custom2), but I havent tried that.

what is the .xnb file? is it generated by the game after you put the custom png and start the game?