ah the reason you are not able to open them is ui_1 and ui_0 are packs, you need to unpack them first so you can see the tga files. The tga files are then quite straightforward to edit with Gimp etc.

See this for now on unpacking:
http://www.twcenter.net/wiki/Unpakin...s:Step-by-Step
info on that is around here somewhere as well, I've lost it again....

sticky at the top of this forum
https://forums.totalwar.org/vb/showthread.php?t=38736
does explain what you need to do instead of re-packing files.