male和female是字符串，因此请用引号引起来！

male and female are strings, so enclose with quotations!

应该是:

<?php if($_SESSION['gender'] == 'female'): ?>
            <img src="img/Refaeli.jpeg"/>
        <?php endif; ?>

        <?php if($_SESSION['gender'] == 'male'): ?>
            <img src="img/avidan.jpg"/>
        <?php endif; ?>