You don't give us much to work with here. Please look in your web server error logs for the error message that the server creates. What error response do you get when you try to display the image directly in your browser? Please read How (not) to ask a question to learn more about how to get useful answers.

My first three guesses are:

• The htdocs location is not served by the webserver as /htdocs but is served as the web root /.
• Your webserver is not configured to execute CGI scripts in the htdocs/test location.
• Your script does not output a valid JPEG image.