PollUnit allows you to configure your competition according to your needs. Regarding security, fraud and access control, you should consider the following aspects when running a photo contest.
Photos that do not meet your criteria can be excluded by activating the approval dashboard. Every uploaded photo can be checked there by an administrator. Rejected photos are archived or deleted and accepted photos are published.
As sad as it is, cheating is not unusual in competitions. How much motivation your participants have to win the contest by cheating depends on your contest. If it's a photo contest between friends or colleagues, the motivation for cheating is probably very low. If you offer prize money, the incentive for fraud is rather high. The more your competition is protected against multiple votes, the higher the hurdle to participate. Our article about the prevention of multiple votes will help you decide.
In addition, you should add terms which clarify under which conditions voting is allowed. You should also mention that cheaters who are caught will be excluded from the photo contest and their content will be removed.