There is a folder under the MovableType installation. It is used to store the files uploaded.
The default is assets_c.
Because of I manually setup the .htaccess file to convert all underscore(_) to dash(-). The link or file under assets_c will be redir to assets-c. And got the 404 error.
So I changed Assets Directory.
1) Make a directory assets-c.
2) Move all files from assets_c to assets-c.
3) Add one line to mt-config.cgi
4) Login the MT system and update all images, etc.
5) Rebuild it.