This is basically the respective issue for what is being worked on for D8 core here: https://www.drupal.org/project/drupal/issues/2061377
This is part of #1087 and #378, and related to #2841

Original issue summary
It would be great to be able to use image styles within CKEditor so that users could get consistent image sizes/styles within their content.
This is a follow-up from #2010 and #2630 where @Dinornis commented:
Maybe the image sizing can be replaced with the option of selecting an Image style predefined in
/admin/config/media/image-styles?
Recent comments
Hi Kevin I am interested assisting you developing a theme by cloning feature from existing WordPress website. Please let me know your suitable time to discuss further...
Create a theme from existing website
I've updated the Zulip link in both places I found it. No need to post again, unless you have something new to say. We'll pull together feedback from all the sources.
Backdrop CMS Core Priorities
Should we post here again, what we posted over there? Or would that unnecessarily duplicate things? The link to a Zulip thread in this initial post leads to an internal one, but there's...
Backdrop CMS Core Priorities