PHP displaying images

Hello I am trying to use GD to create and manipulate images but the statement: header ("Content-type: image/jpg"); generates the error message: The image "http://localhost/gdtst2.php" cannot be displayed, because it contains errors. There is no error message in the ../httpd/error_log file. I am using PHP 5.0.3 with GD 2.0.28. The report from phpinfo() indicates gd present and enabled. Other nongraphics content-types do not gererate errors. Code used was "First Program" from tutorial at http://www.phpfreaks.com/tutorials/105/1.php. Similar code from php manual generates the same error. I suspect the solution is simple but I'm just not seeing it! Help Ian Johnson