This is one of the rarest achievement in Forza Horizon (0.1% on Steam).
1. Access any map in Horizon Realms in free mode.
2. Take a photo of the car.
3. From the main menu, change car (without leaving the map).
4. Repeat steps 2 & 3.
There must be 25 different cars.
The achievement is cumulative (you can take photos on different days).