Default is to store BLOBs (including user avatars) in the database. You can configure Q2A to store BLOBs in the filesystem instead by defining a constant QA_BLOBS_DIRECTORY in your qa-config.php:
define('QA_BLOBS_DIRECTORY', '/path/to/writable_blobs_directory/');
That directory should be outside your Q2A application directory.
The Administration center does not have a setting for configuring this. You must change it directly in the config file.