It sets headers for the image:
From http://wiki.nginx.org/NginxHttpHeadersModule:
[quote]max assigns the time of 31 December 2037 23:59:59 GMT to the Expires header and 10 years for the Cache-Control header[/quote]
This assumes these files will not change, of course. Practically these files will eventually be deleted from a browser's cache as it get filled or flushed.
You can always use:
[code]
expires 30d;
[/code]
if you think these images may occasionally change and retain the same name.
--
Jim Ohlstein