Did you buy the Le Mans add-on pack? If so they go here...
Extract the DDS file to (locate your own location - this is where mine is)...
C:\SteamLibrary\steamapps\common\Project CARS 2\Vehicles\Textures\CustomLiveries
Right now you can only have one active at a time, as far as I know. Hopefully they'll patch this so we can run multiple custom liveries at once (used by ai).