According to the documentation on php.net you will find this line:
Handling Returns: include returns FALSE on failure and raises a warning. Successful includes, unless overridden by the included file, return 1.
Try switching the following:
include 'thumbnail-break.php', '</div> '; ?>
To:
include_once 'thumbnail-break.php';php', '</div> '; ?>
The include_once only returns a true statement, which I don't believe will render in your html. Untested, but let me know how it goes. At the very least you now know why the 1 is emitted.