So ...

Go to Featured Images => Default Images. This is where you can define your own rules to use images as default featured images automatically every time a post is saved:

Here are the available options:

• automatically overwrite an existing featured image while saving a post = this rule will override any featured image set on a post when the post is saved or published, and is extremely useful when you want to set a deafult featured image for each post type, category, tag or author
• automatically use the first content image (if available in the media library) as featured image while saving a post = allows the plugin to use the first image inside a post to be used as a featured image (if no image is found, then the plugin will use the rules you describe on this page)
  

More than that, if you click the "Add rule" button, you'll be able to define your own custom rule based on categories, tags or any other custom taxonomy you may be using on your site. Just select an image, and then choose a taxonomy term. And needless to say, you can add multiple custom rules ...

Finally, let's see the last function ...