WordPress | Add additional image sizes

functions_add-image-sizes.php

<?php
add_action( 'after_setup_theme', 'add_image_sizes' );
function add_image_sizes() {
    add_image_size( 'tiny', 80, 60, true ); // 80 pixels wide by 60 pixels tall, hard crop mode
    add_image_size( 'super', 1600, 1200, true );
}

// Use them like this
if ( has_post_thumbnail() ) { 
    the_post_thumbnail( 'super' ); 
}
// Or
$postid = $post->ID;
$imageid = get_post_thumbnail_id($postid);
$imageurl = wp_get_attachment_image_src($imageid, 'tiny');
echo '<img src="' . $imageurl[0] . '">';
?>

Going beyond Thumbnail, Medium & Large

Sometimes we need more than the three image size options as specified in Settings > Media. Here you'll find the action you need to add to your themes functions.php file to create additional sizes. I've also added some code below to show you how to add them in to your themes template files like single.php and page.php.

Also check out the official code reference here for more information on cropping and sizing.