Hi,
This plugin uses the standard WordPress function get_avatar().
I think by default the avatar is used from gravatar.com. You can change it there for your own email address.
For using a local avatar, you can use a plugin, which you might find here:
https://wordpress.org/plugins/search/avatar/
I do remember some discussion in the WordPress community about offering local avatars in WordPress Core, but I cannot find that discussion right now.