IIS7 URL rewrite map to other external domain

I have a website in which the map "/img" must be redirected to another domain. The other domain can be an external one.

so "http://www.mydomain.com/img/test/demo.jpg" must be redirected to "http://www.otherdomain.com/img/test/demo.jpg"

i also have a <base href="http://www.mydomain.com/">, so the URL to be redirected will be most of the time "img/test/demo.jpg ", which then also should be redirected to to "http://www.otherdomain.com/img/test/demo.jpg"


i currently have this, but it doesn't work

<rewrite>
   <rules>
     <rule name="Change image linking" stopProcessing="false">  
        <match url=".*\.(gif|jpg|png)$" />  
        <conditions>
             <add input="{HTTP_HOST}" pattern="img/(.*)$" />  
       </conditions>  
      <action type="Redirect" url="http://www.otherdomain.com/{C:1}" appendQueryString="false" redirectType="Permanent" />  
    </rule> 
  </rules>
</rewrite>

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How can i do that (in the web.config)

Thanks.