See this tutorial for correct save settings:
https://forums.totalwar.org/vb/showthread.php?t=58962
basically you have to save in dds format DXT5 2dtexture with generate all MIP maps. And with file called name.tga.dds

File you put into models_unit/textures has to be
name.tga.dds
but file you list in DMB has to be
file path/name.tga
if you leave .dds on the end it CTD's when you reach end of battle loading bar!