over the past few days, I have been trying to replace the facepack of a thomas locomotive with my own custom facepack, but I keep running into problems. my current method is opening the facepack in explorer, loading my own facepack into it, deleting the original faces, and renaming my faces to go along with the textures. sometimes that worked, sometimes it didn't. when it didn't work it kept saying "invalid image" or something like that. when that did work however, when I loaded the engine into trainz, I was unable to change the faces. what am I doing wrong here?

ps. I use trainz 2019. the methods for trainz 2019 might be different from other trainz games.