How to delete or autodelete unused images in a Drupal site?
The following drush command:

drush image-flush


may be used to remove all the image style copies generated as image styles via image-field or CKeditor. This is completely safe, as it will not touch the originals.

Those still in use will be regenerated when needed.  Those no longer used will not not be regenerated.

As noted in a comment by cptstarling:


  If you don't have access to drush, you may consider the Image style flush module.