You can take high-resolution screenshots with Alt+P, and these are kept in C:\Users\Username\Documents\BeamNG.drive\screenshots.
@vmlinuz is correct, assuming you're using the in-game photographer. Otherwise, they'd be under a filepath similar to this: C:\Program Files (x86)\Steam\userdata\(numbers)\(numbers)\remote\(numbers)\screenshots No idea what the numbers mean, but steam likes making the screenshot hidden behind dozens of them, for some reason.
You can easily find them by going to Steam > Screenshots. After click one, you'll see a 'view on disk' button, that will open the folder with all of them for that game.
Because the numbers correspond to the game id in the global steam database. It was just easier to make it function this way in the beginning, and if it ain't broken, why fix it?