<?php if (get_the_time('H') >= 8 && get_the_time('H') < 20) : ?>
        <img src="<?php echo get_template_directory_uri(); ?>/assets/images/picture1.jpg" alt="randomalt2" />
   <?php else : ?>
        <img src="<?php echo get_template_directory_uri(); ?>/assets/images/pictur2.jpg" alt="randomalt1" />
   <?php endif; ?>

if (current_time('H') >= 8 && current_time('H') < 20)