You take turns entering a keyword into search, and the winner is someone who can come up with a word for which every image on the first page actually applies.

Rules:

Set search sort for "most recent"
     
Pick a noun, or some other word that is not ambiguous.
   Since some sites use words in the description for search, terms such as "set" are going to be difficult to prove a winner.

Allow for broken phrases.
   If you search for "kangaroo", and see a picture of a landscape with no kangaroo in it, check to see whether it may be a picture of "kangaroo island". 

Everyone takes a turn trying out a word.

If every photo on the first page of results is appropriate, you get a point, and move on to the next round.  If even one image does not actually apply to your keyword, you're out.

Some sites have good keywording, others do not.  The most challenging site is Dreamstime.

Helpful hint:  There are a lot of people who do not know the difference between a macaroon and a macaron.  You'll lose if you try that keyword.

I will play your silly little game if you let me know what the prize is and whether or not it is transferable and/or can be redeemed for cash.