Contestants do not really choose their category of expertise if it isn't available
Obviously the producers need time to gather all of the many (non-copyright) photos, so the categories are pre-selected. I saw on the Reddit post that contestants are allowed to submit their areas of expertise, but if it doesn't match one that's available (or hasn't been given to someone else), they get stuck with a clue.
S2E1-2, Ryan "got stuck" with "Cleaning supplies" and got half of them wrong. This would only happen on the first two episodes filmed on the first day. Episodes 3+ allow the contestants time to study at their hotel.