Here's what I wrote in my notes for Shogun 2.
Code:
Open command window
Extract
stupid.exe extract "C:\Program Files (x86)\Steam\steamapps\common\total war shogun 2\data\data.pack\campaign_maps\sho_japan\display\supertexture\supertexture" shogun2_supertexture 0 0 64 32
Convert (change name of new pack file)
stupid.exe pack "C:\Program Files (x86)\Steam\steamapps\common\total war shogun 2\data\data.pack\campaign_maps\sho_japan\display\supertexture\supertexture" shogun2_supertexture ming_supertexture.pack/supertexture 0 0
I hope this helps.
Also I attached stupid to this post.