[pkmn Platinum] editing trainers help

Hello, I'm new to rom hacking. I've been trying to change the trainer sprite from the trainer card. I've edited my sprite (from the original) and made it into an indexed 8bpp (or whatever it's called) picture. When I try to replace the original sprite it just turns into a scrambled picture.I'm using pokedspic platinum(modified to work for trainer sprites), nitro explorer2, a clean platinum rom and it's narc filePlease help.