Creating a thumbnail from an image already on the server?

Looking at your code, it looks like the file wil be looked for in .../wp-content/uploads/bill.jpeg. You’ll need to include the yyyy/mm as well. I would use this:

require_once(ABSPATH . '/wp-admin/includes/media.php');
require_once(ABSPATH . '/wp-admin/includes/image.php');

$file="http://www.mysite.org/uploads/uploaddir/2012/02/bill.jpeg";

# Function reference
# image_resize ( $file, $max_w, $max_h, $crop = false, $suffix = null, $dest_path = null, $jpeg_quality = 90 )

$wpUploadPath = wp_upload_dir();
$fileName = preg_replace('/^.*?\/(\d{4})\/(\d\d)\/(.*)$/', $wpUploadPath['basedir'].'/$1/$2/$3', $file);

image_resize( $fileName, 200, 200, true, '200x200' );

Give that a whirl and let me know how you make out!