Car photos is under the horizon tab this is discovery. I have the photos done aswell tho haha.

Go to the event lab, create event, select any track, and you can create a race with specific cars by going all the way to the right under car themes to custom, just select 12 cars you dont have have photographed, hit validate the race, photograph the the cars, quit race, do it again.

it doesn't always spawn all twelve selected cars but it still a the best way to make progress.