Need some help with getting textures

Ok, so basically what i need ATM is someone to explain to me what needs to be done to get the unit textures that are allready in the game.

Example:
Say i want to take textures from german imperial knights infantry and change their colors and add some more custom artwork.
What do i need to do to get their(or any other unit's texture) from data file?
And in what way can i make that texture usable once i do what i want with it?

plz help me!!

bump.

Moved to modding questions