These are unpacked from the bsa files. You will not find these files in the listed directories; they are only provided for reference, as you can tell what a certain replacer will replace by comparing the directories. Placing an appropriate texture file in the directory will cause that texture to display instead of the original texture from the bsa. Deleting that file will cause Morrowind to use the original texture packed inside the BSA instead.
Textures added by Tribunal and Bloodmoon are included, at the end of the main galleries rather than being mixed in together.
File Path:
Morrowind\Data Files\bookart
This folder contains illustrations for books in the game.
Morrowind\Data Files\icons
This folder contains all the icons found in the inventory and user interface. There are subfolders as well, for different types of icons.
Morrowind\Data Files\textures
This folder contains the textures of objects in the world. There are a few subfolders, but most textures are just dumped together in this folder.
