By default, Pressable’s origin edge set the cache-control: max-age header to 1 year for static assets.
At this time, it’s not possible to extend the cache duration beyond 1 year or override these headers.
You can verify that the cache-control headers have changed by running the following curl commands for the static asset(s) in question:
$ curl -sI https://149345589.v2.pressablecdn.com/wp-content/uploads/2018/08/logo-new-4x-compressed.png | grep ^cache-control
cache-control: max-age=604800
$ curl -sI https://149345589.v2.pressablecdn.com/wp-content/uploads/2018/08/logo-new-4x-compressed.png?expires | grep ^cache-control
cache-control: max-age=315360000