Normally the way this works (often done on motorcycle boards) is you must take the picture AFTER the challenge is issued and it must be of your vehicle. (doesn't have to be your mk2, but a car you own or are riding in).
So if the first person says "your car with a firetruck", you need to go out and take a picture with your car and a firetruck in the picture. Then you take a picture of your car with something else, and post that up as the example for the next challenge!
Again, the picture must be taken after the challenge is issued, not an old picture from last week or 5 years ago that happens to meet the criteria.
Nothing illegal and please try to keep it to stuff that anybody can find. Also can't be a specific thing, has to be a general thing. Anybody can find a fire truck, but not Chicago FD truck #45, just a fire truck.
Here are some examples of things that make good challenges:
creepy old industrial building
pier (close enough)
ice cream maker
observation post/one room school house
cool old diner sign
tool booth photo by attendant
moose crossing sign