I was taking this quite literally and directly: I can photograph water, fire and earth quite easily; but air can only be photographed "indirectly". (Which is why it's perhaps the most interesting.)
I guess that's ok, because the challenge does say: "Create an image representing your choice of Earth, Air, Fire, or Water."
So the word "representing" is the key to this. I imagine you could even photograph a symbol of any of these (though you may or may not score well)!
And tragicharpy, it does say "choice" which means one or more.
