Add images from external URL in WYSIWYG Editor
This question was answer in the Drupal issue queue - https://www.drupal.org/node/2119379


  Reusing existing images will be possible in Drupal 8 contrib by
  overriding the managed_file element, which is the element that renders
  the image upload button. E.g.
  https://drupal.org/project/filefield_sources does that: it provides a
  "Reference existing" option.
  
  When you disable image uploads, you can insert image URLs. It's either
  of the two, not both. If you want both, then again something like
  https://drupal.org/project/filefield_sources provides the solution.