Forza Horizon 2

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+

Happy hunting!

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!

The best way to get this achievement is to switch to photo mode at the start of each race area in order to capture at least 4 cars per picture and you could get up to 12 unique cars depending what's on the track per race. That's while they're bunched right at the start of the race get as many into each shot as possible, should be quick.

An other good way is to take the highway that links Montellino & Sisteron. You will meet a lot of cars and get photos easily.

Here's a map, the highway is the green line (thanks to XBU Philippe)

This is only add-on information as its not mentioned after you continue taking pictures after achievement. You still get rewarded, and more than before the achievement.
You get still 1000 CR for every car, but tier bonuses increases even more.

Some tips:
- First of all, get the perk that marks all cars you don't have taken photo of.
- You don't need to save any photo ever, and the cars barely have to be visible on screen when you take the photo.
- Note that one photo can register 5 unique cars on screen at the same time. Happens frequently when you take a photo on start of each race. Remember to turn 180 degrees to take a photo of the cars behind you too.
In one race i got as much as 7-8 cars registered on only 2 photos at the start of a race.
- You can take picture of parking lots and get photo rewards, and also for unmarked unique AI cars. AI cars is never marked, but if they are unique, you get same reward as on drivatar cars.

When you have taken 240 photos of unique cars, you get 50.000 CR.
When the goal is 260 photos you get 100.000 CR.

280 Photos - 100.000CR
300 Photos - 100.000 CR
320 Photos - 100.000 CR

I don't know if it ever ends. Looks like som different cars skins also counts sometimes.