You unlock Horizon Promo once you arrive at Montellino for the first time during the second roadtrip. The idea is to take photos of as many different unique cars as possible. After every 20 cars you will receive a Credits bonus.
For the achievement you will need to take photos of 100 different cars. Here are some tips to get the achievement faster:
- At the start of each new championship take a photo of the grid in the first race (in front and behind you)
- Every time you discover a Bucket List car take a photo of it
- Traffic cars count towards the total so when you're on a busy road take some photos of the traffic
There is a perk available for 3 Skill Points which will put a camera icon above any cars you haven't captured which really helps out when you get to 80+
Update 1 - You do not need to save each photo. Once you have seen the screen which lists each new car, you can just press B to exit.
Update 2 - Horstwayne points out that Roadtrip events will net you a good amount of new cars too!