Forza Horizon 3

Photo mode option is available by unlocking in the 'Skill Shop'.
You can find the Skill Shop in the pause menu under the tab 'Progress'.
Under 'Festival Boss' in the middle you can see 'Horizon Promo', this is the one you need to unlock.

You will need 3 skill points for 'Horizon Promo' itself, but 3 more for 'Drone Mode' & 'Horn Import'. So total 6 skill points to get to 'Horizon Promo' and unlock it.

You can open 'Photo Mode' in two ways:
- From the pause menu , in the home screen bottom left above 'Drone Mode'.
- By holding up on the d-pad .

You will see a photo icon above other festival drivers (offline mode) if you don't have a picture of that car yet. However it seems to not show when the driver is a Drivatar player (The 'D' icon). So it is possible that you do not have a picture of the car yet and isn't showing. This is also a possibility for 'normal' drivers. It might be worth in to snap a few random pictures of both.

The total amount of available cars where you can a picture from are 425. You will need 300 for this achievement.
Do you want to know which cars you are missing? In freeroam go to 'Progress' in the pause menu and select in the top right 'Horizon Promo'.

Tips and tricks:
- To snap several cars at once - pause after starting a race.
- It is possible to get new cars when restarting a race.
- Choose races with varied line-ups of mundane cars. There is usually more variety.
- Other cars on the road seem to be adjusted to the class/type of car you're driving.
- Street races are really good for this.
- It will also count if you take a picture of a Bucket List car when you see it on the spot to start the Bucket List.
- It WILL count if you take a picture of a car in garage mode (see below).

Taking pictures in 'Race mode'
Start the race, switch to photo mode right away. Snap pic(s) of all the cars.
Exit photo mode and the quit the event.
Go back to the same site you set up the exhibition race. Choose to edit the blueprint to the exhibition. Choose a different style of car and rinse and repeat.

When creating an Exhibition Blueprint, you can individually select up to 50 vehicles that are eligible for the race. If you keep track of what cars you have and haven't taken photos of you can select 10-11 that you need, start the race, snap a photo, then quit the race.
Keep in mind it will choose cars based on eligible vehicles around the PI of the vehicle you drive, so if you choose a bunch of S1 class vehicles to go with the C class vehicle you are driving, all the drivatars are likely to be in the same vehicle as you.

The only vehicles not available this way are traffic cars (BMW 323ti, '04 Audi S4, Ford Transit Diesel, etc; some vehicles have a traffic specific model and one you can drive, the traffic version will have a license plate on the front when browsing through your Horizon Promo progress list.) and pre-order cars that you have not entered a download code for (although these will occasionally show up in free roam and races setup by car class).

Taking pictures in 'Garage mode'
Pick a car from your 'My cars' in the garage and select it to drive.
Tab left to 'Autoshow' and press on Forzavista, then press to snap a photo.
If you have it, it'll say no cars and if you don't have it, then the car will show up on the menu to the left.

Special thanks to lesleylion, AussieBigfoot, Cajeel, AH GamerScore, SockGORE828, dolphanatic88, Borg564 & SneakyStabbalot!

Comments and additions to this guide are appreciated.
Have fun Down Under!

[Last update 18-Oct-16]

I'd also like to add to the suggested solution above. I also took pics of my own cars, barn finds, etc.
It's a little time-consuming, but it can be done a slightly different way as well. If you blueprint a race as an "Anything Goes" race (i.e. - you can use any car you want), then you can just set up a race, as soon as it starts, enter photo mode and then take pictures of the cars. Back out, change your car for the race, do it again. On average, you'll get 5 cars/race, sometimes more, sometimes less.
Halfsies credit for this idea goes to Rayzor Nippler.

First up, thanks to "SantaUndercover" for mentioning where to find the "Not Captured" vehicles.

The best way I've found to photograph almost any vehicles is a s follows:

STEP ONE (1-5) - MAKE A LIST:
1. In freeroam, press the pause button.
2. Press “RB” until you reach the “PROGRESS” tab.
3. Select “HORIZON PROMO”.
4. Press “Y” and select “NOT CAPTURED” and press “B”.
5. Make a list of the vehicles you need to photograph.

TIP: Simply write out the list if only a few vehicles are required. If you need quite a lot of vehicles however, use your smartphone to voice record the name of each vehicle into an email, SMS or similar as you scroll through the list. This can then be printed either from your phone or copied into an email which can be sent to your PC and printed from there (editing if needed). This enables you to tick off each vehicle as you photograph it.

STEP TWO (6-12) - CREATE CUSTOM EXHIBITION:
6. Drive or fast travel to any racing event and press “X” to “SETUP EVENT”.
7. Choose “EXHIBITION”.
8. Choose “BLUEPRINT AN EXHIBITION”.
9. Press “RB” button until you get to “CUSTOM” and press “A”.
10. Scroll through the vehicles and press “A” to add any you are missing for the achievement.
11. Add up to 49 vehicles that you require + one vehicle that you own (so you can take part).
12. Press “B”, select “YES” to confirm selection and select “CREATE BLUEPRINT”.

STEP THREE (13-25) - TAKE PHOTOGRAPHS:
13. Press “X” to “SETUP EVENT” and select “EXHIBITION”.
14. Select the “EXHIBITION” you created.
15. Select a vehicle and once loaded select “START RACE EVENT”.
16. When the race has started, immediately open photo mode and take photos as normal.
17. When processing is complete, press “B” twice and select “YES” to “EXIT PHOTO MODE”.
18. Pause and “QUIT EVENT”.
19. Drive back to the same event and press “X” to “SETUP EVENT” again.
20. Choose “EXHIBITION”.
21. Choose “EDIT YOUR EXHIBITION” and press “A”.
22. Press “Y”, select “Selected” and press “B”.
23. Deselect any vehicles you’ve already photographed.
24. Press “B”, select “YES” to confirm selection and select “CREATE BLUEPRINT”.
25. Select “YES” to “OVERWRITE EVENT”.

STEP FOUR (26-27) - RINSE & REPEAT:
26. Re-enter the “EXHIBITION” you created as different cars from your list will be in the next race.
27. Repeat 19-24, deleting any photographed vehicles so they don’t appear again.

TIP:If your wanted vehicles stop spawning after a few races, change your own vehicle at your garage and add it whilst in the “EDIT YOUR EXHIBITION” and this should spawn different vehicles.

Reading the above it seems very laborious, but that's because I've listed virtually all button presses for anyone starting out. It doesn't take long in reality.

Just before I started writing this, I needed 40+ vehicles to complete the achievement and whilst testing, I managed to photograph 20+ vehicles in 5 races, which is something I hadn't seen since the start of the game. :)

Used the same technique on my son's account and got ALL 25 required vehicles in 3 race starts. :)

I ended up buying one of the last six vehicles I needed in the "Auction House" and when I used that car in the same custom exhibition, 4 of the last six vehicles where on the start grid! LOL

I was struggling to find just two cars in order to have all 539.

One was the 2016 Ford Shelby GT350R Pre-order Car, which I ended up seeing randomly driving around in the desert (as you do).

The other was the * 2011 Porsche #45 Flying Lizard 911 GT3 RSR, which was a bit more elusive for me.

In the the end I tried creating custom blueprint exhibition with just two vehicles.

These were the #45 Audi R8 and #45 Porsche 911 "Flying Lizard" cars and when I started the race the lineup was 50/50 Audi/Porsche, so managed to capture the last car.

Not needed for the achievement as that kicks in @ 530 cars photographed,, but if you have slight OCD and want to capture all of the cars, maybe this will help someone.

You can team this up whilst working through the completing every Championship achievement as well. When doing an event change car for each race and you will get different cars appear in the next race. With the variety of the Championships you should get there pretty soon.

Also when taking a photo at the start of a race make sure you take a photo in both directions, sometimes you can't get all the cars in one shot!

easiest way i've found to get pictures of the preorder cars if you don't have them:

mercedes-amg c63 s coupe preorder - get into the regular version of the car and blueprint an exhibition race with the following settings: current car > AMG. select the regular version of the car when starting the race, and the race will be filled with regular and PO versions. the unupgraded version of the car seems to prevent the other AMG's from qualifying.

for the jaguar - get into the regular version and auto upgrade to s2, blueprint an exhibition with current car > jaguar and cross your fingers. i tried twice without doing the upgrade and had no luck. doing the upgrade made less cars qualify for the race, giving me a better chance at getting a PO car.

Can be unlocked within 10 minutes.
You should run for this achievement after you have atleast 100 cars, fully upgraded all festival sites (achievement) and collected 15 barn finds (achievement).
Create a blueprint exhibition with an active category of type of car, for example "hyper cars".
Choose a car from that category, start a race, as soon as you start - take a photo of the cars.
Then exit, edit your blueprint to another car type (for example retro cars), and repeat.
I unlocked it in 10 minutes :)