Put the new skin into Data/models_unit/textures
If there is no such folder, you must make them.
Then the entry should be:

type carthaginian_infantry ; combat spear
skeleton fs_swordsman
indiv_range 40
texture carthage, data/models_unit/textures/unit_carthage_infantryreskinned.tga
texture spain, data/models_unit/textures/unit_carthage_infantry_spain.tga
texture slave, data/models_unit/textures/unit_carthage_infantry_rebel.tga
model_flexi data/models_unit/unit_carthage_infantry_high.cas, 15
model_flexi data/models_unit/unit_carthage_infantry_med.cas, 30
model_flexi data/models_unit/unit_carthage_infantry_low.cas, 40
model_flexi data/models_unit/unit_carthage_infantry_lowest.cas, max
model_sprite slave, 60.0, data/sprites/slave_carthaginian_infantry_sprite.spr
model_sprite spain, 60.0, data/sprites/spain_carthaginian_infantry_sprite.spr
model_sprite carthage, 60.0, data/sprites/carthage_carthaginian_infantry_sprite.spr
model_tri 400, 0.5f, 0.5f, 0.5f