folders are 755, files are 644, right?
but when I upload picture, I got message saying can’t create directory in wp-content folder. I have to change the wp-content to 777 to upload, then, change it back after upload. is this right?
Change the permissions on the wp-content directory to 766. If you still have problems, try 767 or 777. Once your uploads are working, change the permissions on wp-content back to 755 again and check that everything still works OK. http://codex.ww.wp.xz.cn/Changing_File_Permissions