Have a look here http://www.phpjabbers.com/phpexample.php?eid=20 http://lmgtfy.co.cc/?q=watermark+using+php
LOL! I accept that! I thought that the Thread Starter asked for a Stand Alone PHP Script and not an integration module with WP or Drupal
Infact I also thought that OP wants a Stand Alone watermark PHP Script..... Thats why, I posted "You can also use any CMS like WordPress with watermark plugin or Drupal with watermark module."