If you have your own S3 account you can configure it in the config/config.json file. You should just need to go to the AWSSettings section and fill in the follow fields:
Once those are configured properly you should be able to start uploading/downloading images and files to your S3 bucket through your Mattermost instance. (Make sure to restart your server after filling these in).
As it is now we don’t have any way to allow you to configure local server storage, but this is something that’s on our roadmap of things to add