You can override game's textures with images in DDS (recommended) or PNG format.
To know where to put your custom textures, set the option trace_loaders to true and look at the content of FFNx.log
when the game is running.
It is also possible to dump textures on runtime in PNG format by setting the option save_textures to true.
By default, FFNx looks for textures relatively to the ./mods/Textures directory, this path can be changed via the option mod_path.