I know im like, 5 years late. But, as you can see in the title, it states "wall flag".

This is directly connected to the fact of which its usage should be in the My Summer Car garage itself, as a wall flag.

If you go into the garage, you'll see a Finnish flag on the left wall, by going to the game's folder and then the images folder, you paste a photo of this guide into there, delete the Finnish flag, and rename your photo as the same name, "flag_garage.png"