If you use Addon Spawner by IKT : https://www.gta5-mods.com/scripts/add-on-vehicle-spawner

drop the picture in this location:

Grand Theft Auto V/AddonSpawner/img

If you use Menyoo trainer by ItsJustCurtis : https://www.gta5-mods.com/scripts/menyoo-2-0 

drop the picture in this location:

Grand Theft Auto V/menyooStuff/Graphics/vehiclePreview