help with importing textures into iff files

[Rodzilla91]

dont know where this sort of post really goes, but im wondering if anyone could help me on how to properly upload textures into iff files,

lets say (for example)

i use jimmer fredettes face file, and i make the hair color texture darker in GIMP, export to dds, and then import it using WINRAR (as you can open iff files, using winrar), and whenever the face pops up during the game (i have it on my myplayer, so as soon as the game boots up into the menu), it freezes, then crashes. is there any way i can CORRECTLY import it or fix any of the steps ive done? i dont really want to import textures into the file in the game archive, because i dont want to replace files, minor edits or not, as i just end up exporting the iffs from the archive and use the "Modded" folder.