The avatar for the administrator comes from the administrator profile at gravatar.com so if you haven't set a gravatar for yourself you can simply go here and make one which suits your face or work. This will go the same for your users in Lift. Their gravatar will be displayed as their Lift's avatar.
But if you intend to make custom avatars for each of the administrator and users panels as default you can easily change the avatar in Lift File Manager with below steps:
1- Open the filemanager_config.php from your root directory where you have installed your Lift File Manager on your server.
2- Then simply enter the image urls to be used for default admin and user avatar in 14 and 15 of the mentioned file.
* Custom Avatar
//define( "ADMIN_DEFAULT_AVATAR", "http://url-to-your-image.com/your-image.png.jpg.gif" );
//define( "USER_DEFAULT_AVATAR", "http://url-to-your-image.com/your-image.png.jpg.gif" );