Function to auto-resize images already uploaded to WordPress.

gistfile1.php

/* 
 Place inside your functions.php 
 Example usage inside template: echo jt_get_image(get_the_ID(), 'my-image-size');
*/
    function jt_get_image($post_id, $size='post-thumbnail')
    {
        global $_wp_additional_image_sizes;

        $thumb          = get_post_meta($post_id, "_thumbnail_id", true);
        $fullsizepath   = get_attached_file( $thumb );
        $filesource     = wp_get_attachment_image_src($thumb, $size);

        if (!$filesource[1] || !$filesource[2] || $filesource[1] > $_wp_additional_image_sizes[$size]['width'] || $filesource[2] > $_wp_additional_image_sizes[$size]['height'])
        {
            $metadata = wp_generate_attachment_metadata( $thumb, $fullsizepath );
            wp_update_attachment_metadata( $thumb, $metadata ); // regenerates all thumbnails for this image
        }

        return wp_get_attachment_image($thumb, $size);
    }

The problem with using WordPress's "add_image_size()" function is that the new size is not applied to any existing images.
Use this function if you've added/created a new theme and need to resize previously uploaded images on the fly.
The resize portion should only be applied to the image if the required image size doesn't already exist.