Edit Skin Portraits?

How do i do this? like i wanted to edit the captian skin so it woud be fully high elf swordsman skin is to make the swordsman to have not have eyeballs, just plain white but when i opened the skin the captain already had none eyeballs but in the game he still has them.

Most of the units have blank eyeballs, and they have another small circle texture that's added in. Look around on the skin for a little sphere, usually surrounded by alpha channel.