For CHALLENGE RULES they are on the Challenge page under WHAT IT IS. Sometimes there are additional rules so check the ADDITIONAL RULES tab on the Challenge page.
For general rules that also apply go to the top navigation bar and see ABOUT > WEEKLY CONTEST RULES
I found out the member challenges are moderated by the member who starts the challenge. A couple of my images got disqualified because some members have a narrow localized view on things. For example a rural "Country Memories" excludes images that have a beach or ocean. Even if you're country memories are from growing up on an island. Nothing you can do about that type of disqualification.