First you need a tool called TXD Workshop. I assume you already have it so.
Open your TXD that has your models textures in it with TXD Workshop.
Then for each texture, go into properties and then click a tick box called "compress".
If you havent made a TXD yet, Go to TXD Workshop and create a newtexture and then import the textures from your car
Then go to the properties of that texture and click that compress box.
Duplicate this texture and then import the other texture from your car (if any)
Keep doing this until you have done all of your cars textures.
Then go to the File menu in TXD Workshop and save Txd
Name it the same as the car you are replacing and then replace the cars txd within the IMG Tool.
Didnt really know how to explain it.
Hope this helps