User meta keys

In wp_usermeta table, see meta_key wp_user_avatar
for multisite, wp__2_user_avatar etc.

Note that the meta_value is a number, the ID of the media library item of the photo.

Note, however, it is preferable to use an API call:
https://codex.wordpress.org/Function_Reference/get_avatar