Which program did you use for watching the cars? I do it in race.
My method is easy, and dont require any heavy program in the background.
Download the x-file of the car you paint from GTR4u.de..
Then download Carview from this post..
All you do is extract the xfile to a convenient location where you can browse quickly with your graphics editor,
then just overwrite correct texture and hit refresh textures in carview.