I assume you're specifying the image placement and dimensions using the popup up bubble with icons within the RT.
Unfortunately, this bubble must be considered as Settings. Like the settings in the settings tab. So it's not CSS per se, it's settings, hence not being responsive. Change it for a breakpoint and it'll affect all of them.
However you can style the
<figure> element and it will be responsive.
Select your image in the rt and go to navigator tab to go up in hierarchy to select the figure element. then click on the class field, select Style all Figure elements, then limit to the class of the rt just under.
Now fond a way to style the figure as you want, for desktop, devices.
If you have trouble to do that, comeback with your sharing link so I can try too.
1 - On your site's dashboard click the Share icon:
2 - Generate or Copy the link:
3 - Then edit your post here and paste it inside.