Postgres optimizations should inhance the instance speed and loading time for all elements on the server itself and inhance loading the local time line.

Content delivery on the other hand have to do with the data center and the disks used for that instance. If you really have a good uplink for the server and the disks are also good, images will load faster. Otherwise nothing would change. It also related to how much IO is going on and how much you're utilizing the disk at the time the image was requested.