Issue making mod

I'm trying to make a van texture mod myself by editing an existing van mod, which DOES work in game.
So I take it, convert from .TEXTURE to .dds, edit what I want to edit, save as .dds and then rename back to .TEXTURE
However, after I make my edits, my mod doesn't seem to work in game. It detects in the mod list but it simply shows the game default van. Does anybody have an idea that could help me out?
Replies
Sorry, I jumped the gun a bit.
No, I was recommended Paint.net, so that's what I'm using. Should I switch to Photoshop?
No, I do not.
I am absolutely positive I am editing the correct van.