Hi argie01. Sorry I missed your questions.
#1 Actually default image can be whatever you want if you specify URL. I added this as a callback if your post has no featured image. I do not recommand you to leave this field blank but indeed you do not have to keep the gravatar image. Just enter your own URL for your own image.
#2 No this plugin is not meant for this. You can’t do such a thing without authentification by the way. So you’ll have to install another plugin.