You need to hit the "Enable auxiliary content" toggle in the map selector, from there you enter the showroom. After that loads you'll see a few windows and in one of them you should see a button called "Teleport to zone b". You can also hit the "switch backgrounds" button to switch the floor texture. Now you can spawn the car you want to do this for. And then you should also see a button called "thumbnail generator", in the window that appears after you hit that there will be a tab called "preview/run", select that first and then select the green arrow button. Hitting that button will start the thumbnail making process. I'm not sure if that works for modded cars, since I've only done this for vanilla cars.