看来我需要
<rule name="WordPress Rule 1" stopProcessing="true">
<match url="^index\\.php$" ignoreCase="false" />
<action type="None" />
</rule>
<rule name="WordPress Rule 2" stopProcessing="true">
<match url="^([_0-9a-zA-Z-]+/)?files/(.+)" ignoreCase="false" />
<action type="Rewrite" url="wp-includes/ms-files.php?file={R:2}" appendQueryString="false" />
</rule>
这里还有我能找到的更多信息。
http://wordpress.org/support/topic/images-files-not-displaying