In order to vote you need to provide a valid email address, and only one vote per email address is possible. If after voting the first time, you decide to vote for another image, the first image you voted for will lose that vote.
The "People's Choice" is a section apart from the rest of the contest and the votes will not influence the judges. A photographer would seek votes in order to win the "People's Choice" section alone. The winners for the other categories will be determined by the judges.