Sorry, they used to post the rules on every photo challenge. Here's an old one with the Rules listed about halfway down:
https://steemit.com/steemitphotochallenge/@jamtaylor/steemit-photo-challenge-10-theme-announcement-sep-22-25

But I'll answer the questions as best I can.

1. Each challenge usually starts on Thursday. Deadline usually midnight Sunday UTC -4 (For me that's 9pm Sunday Pacific Time). They announce the winners mid day Monday.
2. JamTaylor, the host, or a selected guest judge will pick the photos that best represent the topic of the week. They definitely encourage you to take new photos relevant to the contest as opposed to using one of your photos you took 5 years ago. The link above will describe more but they base their judgments on concept, composition, technical, etc. Go click on the host JamTaylor's page to see his previous posts of winners to get an idea.
3. New topic announced each Thursday. They've already done architecture, landscapes, city life, animal, fashion, etc. You don't know what the next topic will be until they announce it on Thursday.
4. You can only submit a maximum of three photos per weekly contest. If you submit more than three then they'll only consider the first three.
5. Yes, if your photo is a winner or an honorable mention in the contest, they will give you credit and will send you some Steem, too. And if you win an award, it helps your post gain some $teem payout and followers.

Make sure to read the full rules. The new challenge should be out today (I still need to check it out).