I want to improve my urls for seo

I'm with rails attachment_fu

I want change this url

/attachments/000/234/my_great_image.jpg

like that :

/my_keywords_seo/images/my_great_image.jpg

I tried it

location ~* ^/attachments { rewrite ^/attachments/([0-9]+)/([0-9]+)/(.*)$ /my_keywords_seo/images/$3 last; }

the url is rewritten, however, the resource is not found !

However i read in http://wiki.nginx.org/NginxHttpRewriteModule

Flags can be any of the following: last - completes processing of rewrite directives, after which searches for corresponding URI and location

I don't understand ...

Somebody can help me ?