fanctions.php

// Add Thumbnail Theme Support
add_theme_support('post-thumbnails');
add_image_size( 'toppage', 480, 273, true );
add_image_size('large', 700, '', true); // Large Thumbnail
add_image_size('medium', 250, '', true); // Medium Thumbnail
add_image_size('small', 120, '', true); // Small Thumbnail
add_image_size('custom-size', 700, 200, true); // Custom Thumbnail Size call using the_post_thumbnail('custom-size');

テンプレート

<?php if (has_post_thumbnail()) : ?>
    <?php the_post_thumbnail('toppage'); ?>
<?php else : ?>
<?php endif; ?>