WordPress: Generate thumbnails on-the-fly

otf_get_attachment_image_src.php

<?php

/*
 * Check for thumb of $size and generate it if it doesn't exist.
 *
 * @param int $post_id Post ID for which you want to retrieve thumbnail
 * @param string $size Name of thumbnail size to check for; same as
 *     the slug used to add custom thumb sizes with add_image_size().
 * @return array An array containing: array( 0 => url, 1 => width, 2 => height )
 *
 */
function otf_get_attachment_image_src($post_id, $size = 'thumbnail', $force = false) {
    $attachment_id = get_post_thumbnail_id($post_id);
    $attachment_meta = wp_get_attachment_metadata($attachment_id);

    $sizes = array_keys($attachment_meta['sizes']);
    if ( in_array($size, $sizes) && empty($force) )
        return wp_get_attachment_image_src($attachment_id, $size);
    else {
        include_once ABSPATH . '/wp-admin/includes/image.php';
        $generated = wp_generate_attachment_metadata(
            $attachment_id, get_attached_file($attachment_id));

        $updated = wp_update_attachment_metadata($attachment_id, $generated);

        return wp_get_attachment_image_src($attachment_id, $size);
    }
}