A car's skin starts as a template generally made by whoever made the vehicle. That template file is named like: car_xxxx.psd
.psd , stands for Photoshop Document.
The skin is created in in programs like Photoshop but a .psd file needs to be saved as a different format for the game.
That is the .dds format, DirectDraw Surface. A dds file cannot be edited the way a psd file can so the golden rule of skinning is, save as psd before dds.. Then you can still change it.
The game can display the dds of your skin but if you want your custom window banners or helmet etc, you skinned then they all need to be packaged in a mas file.
That is where the free tour ends and remember to take your children with you as they are extremely hard to sell are valued many times less than you assume is market value...