Use responsive images inline
The module Entity Embed will be able to do that. There is now a dev version available, but you better wait for the first alpha/beta-release.

Even longer it will take, until this is in core:

Allow image style/responsive image mapping to be selected in Text Editor's image dialog

This is postponed to drupal 8.1

Until then you can use jquery to move the image from an image field to an empty container in the body. To avoid flicker effects, add a style display:none in the twig template of that specific image field and show it after moving it into the body in jquery.