If you are taking pictures that are new since yesterday afternoon, it's not something you need to worry about. Just have fun and post your pictures as you usually do. There is nothing else you have to do.
If you're in the running as a winner, you might have to show that your pics were indeed all new, just because it is a contest and some people may either not have been aware of the rules or thought they'd skirt them a bit
(as my DH and his team did the time we did a scavenger hunt in real life; of course he would also tell you that one of the people on my team tried to stretch the interpretations a bit and I wouldn't be able to disagree, lol).
Remember, it's all in fun. Now stop worrying and go out and get some pictures.