Thanks a lot.
I got the problem fixed.
This is the error message I never readed:
2009/06/04 20:21:37 [crit] 6390#0: *1 open() "/usr/local/nginx/fastcgi_temp/0/00/0000000000" failed (13: Permission denied) while reading upstream, client: xx.xx.xx.xx, server: xx.xx.xx.xx, request: "GET /xxx/4.php HTTP/1.1", upstream: "fastcgi://127.0.0.1:9000", host: "xx.xx.xx.xx"

I guess the reason is because of some directories belong to apache, some other directories belong to root which are created before. So nginx can not use some of the temp directories to handle parts of the image.
I chown for /usr/local/nginx/fastcgi_temp/ and problems gone!