The Photographer Achievement is awarded for taking 100 photos of different Cars for the Horizon Promo Collection - an effort that you can easily accomplish during casual driving thanks in no small part to the Skill Points Perk (ability) called Paparazzi, which gives you the ability to spot cars that are part of the Promo Collection and which you have not yet photographed thanks to the Camera Icon that appears above them and below the Drivatar Name.
So as you cruise the highways and byways of the lands in France and Italy, you should easily be able to spot at least the Cars you need that are driven by Drivatars - not all are by they way, as some of the cars you need are actually driven by civilians, so even this Perk Ability will not help.
The Casual Approach
Before You Begin to work towards this Achievement you have to decide whether or not this is going to be one of the ones you grind out, or whether it is one you will take as it comes.
Personally I prefer the take as it comes route, because it allows you to casually snap pics as you encounter Cars on the road that have the camera icon above them. It takes all of the pressure off of it - you get them when they are available, and otherwise you don't think about or worry over them. That seems like the best approach to me!
As you can see from the video above, I took that approach and as a result got what I needed when it was available but did not otherwise stress over it. Bearing in mind though that 100 Cars - 100 pictures - is way less than half of what you actually need to complete the Promo Collection - so in a way it is more of a taste of that collection than it is a reason to celebrate it.
That said, we strongly recommend that you unlock the Paparazzi Ability as soon as you can after starting the Horizon Promo Collection so that you can take the casual approach with ease.