Change/update the default user profile photo/thumbnail folder location
In Craft 3 the place that users photos get stored in are proper Asset Volumes, meaning they can be stored in a folder on the local file system, or on S3, Google Cloud, or any other place that a plugin provides a Volume for.

Once you create a Volume for user photos, go to Settings->Users->Settings in the Control Panel and you can choose that Volume under the "User Photo Volume" settings.