ok im ganna try i hope it works
I just checked out the trucker's cap in the player.img file because I had a hunch and sure enough I was right... Your problem is probably due to the fact that the texture graphic has an alpha channel and importing/exporting as a .bmp doesn't include one. When an image inside a .txd file has an alpha channel attached to it, you'll see an extra name for it under the main one in Txd Workshop's listing of it. For example, it says:
Name: "captruck" (main graphic)
alpha: "captrucka" (alpha graphic)
When you see this, it means there is a separate image that's attached to the first in it's alpha channel. It's a 256-level greyscale image that defines areas of transparency in the main image. Total white = opaque. Total black = transparent. It's like a cookie-cutter for images.
To work with this sort of image, you must first export it as either a .tga or .png format. These filetypes handle alpha channels. Next, you need a paint program that can work with alpha channels. Whether Paint.net does or not I have no idea cuz I've never used it. Then you need to save your image in one of these types and import it as such.
For more info on this topic, search for an earlier post I made about it or search the net. I'm not going to go into it any further here... There's tons of info elsewhere - that's how I learned!