Destac on "[Plugin: W3 Total Cache] Nginx server, Speed Test says images have short freshness lifetime"

I have an Nginx server setup, and I'm sure W3tc is writing to the .conf file, and that said .conf file is included in the main server's main nginx.conf file, so that the settings should be getting picked up.

For the browser cache settings, I have 31536000 seconds (1 year) set for the Expires header lifetime for media.

And yet, when testing on pingdom's website speed test, it says images have a "short freshness lifetime. Specify an expiration at least one week in the future" ... so, apparently, they are not even set for a week.

What could be wrong here? Thanks for any help.

https://wordpress.org/plugins/w3-total-cache/