Did you add error checking to the open call? Sounds like that's failing if uses of PNG are generating errors afterwards. It may be permissions. Remember that your CGI runs as whatever user your Apache httpd.conf has the web server set to (default is nobody).

But if there are errors like that I'm not seeing how you're getting the image to show unless you created it successfully outside of the web server environment (e.g., running your CGI manually) once along the way.