Change Default Thumbnail Sizes in WordPress Easily

Code

<?php
/**
 * Change default thumbnail sizes in WordPress.
 *
 * This function modifies the default image sizes for thumbnails, medium, and large images.
 * It should be added to your theme's functions.php file.
 *
 * @return void
 */
function wp_dudecom_modify_default_image_sizes() {
    // Set the default thumbnail size
    update_option('thumbnail_size_w', 150); // Width in pixels
    update_option('thumbnail_size_h', 150); // Height in pixels
    update_option('thumbnail_crop', 1); // Crop the image to exact dimensions (1 for true, 0 for false)

    // Set the default medium size
    update_option('medium_size_w', 300); // Width in pixels
    update_option('medium_size_h', 300); // Height in pixels

    // Set the default large size
    update_option('large_size_w', 1024); // Width in pixels
    update_option('large_size_h', 1024); // Height in pixels
}
add_action('after_setup_theme', 'wp_dudecom_modify_default_image_sizes');

/**
 * Regenerate thumbnails after changing image sizes.
 *
 * This function hooks into the admin_init action to ensure thumbnails are regenerated
 * whenever the image sizes are modified.
 *
 * @return void
 */
function wp_dudecom_regenerate_thumbnails() {
    if (function_exists('wp_schedule_single_event')) {
        // Schedule a single event to regenerate thumbnails
        wp_schedule_single_event(time(), 'wp_dudecom_run_regenerate_thumbnails');
    }
}
add_action('admin_init', 'wp_dudecom_regenerate_thumbnails');

/**
 * Run the regenerate thumbnails process.
 *
 * This function is triggered by the scheduled event to regenerate thumbnails.
 *
 * @return void
 */
function wp_dudecom_run_regenerate_thumbnails() {
    if (function_exists('wp_create_image_subsizes')) {
        // Get all attachment IDs
        $attachments = get_posts(array(
            'post_type' => 'attachment',
            'post_status' => 'inherit',
            'posts_per_page' => -1,
            'fields' => 'ids',
        ));

        // Regenerate thumbnails for each attachment
        foreach ($attachments as $attachment_id) {
            wp_create_image_subsizes(get_attached_file($attachment_id), $attachment_id);
        }
    }
}
?>