I'm actually checking the closest mod I could find that changes trainer sprites, which is: https://forums.pokemmo.com/index.php?/topic/149186-mod-npc-boss-sprite-mod-v11new/#comment-1928329
And apparently inside "Sprites" there is "trainer" and then 0, 1, 2, 3, with the trainers sprite updated. I see they all have the name like as if they're "pokemon", so I just need to find the number correlated to our own trainer and make a new png for it, if I understood it correctly.