I've added a few skins into the "assets\pixelmon\textures\trainers\steve" directory, but I think they have to also be registered in a config somewhere. I'd think a config detailing the example below would exist for us to implement our own trainers, but I don't think it exists yet. Any confirmation?
#1 Trainer Name=Name Here
Skin: Name.png
#2 Trainer Name=Name Here
Skin: Name.png