Do you mean that the picture got posted actually, just he got a 503 error page? May be the PHP process crashed during post processing?
If possible, please setup another virtual host on a different port, enable debug logging for that virtual host, reproduce the bug and send us the log file. It probably is the best way to identify and fix the problem.
Maybe you should upgrade to PHP 4.3.9 with GD 2.0.28. :-)