Create User Picture Field
You'll find the configuration files of the user picture field in the standard profile. You can export the complete config, add the yaml files of the field and reimport again.

Or if you want to import the field in UI, go to admin/config/development/configuration/single/import, select the configuration type Field storage and import the content of 

core/profiles/standard/config/install/field.storage.user.user_picture.yml

The same for the configuration type Field and the content of

core/profiles/standard/config/install/field.field.user.user.user_picture.yml.