You can add padding to the inside of the image. You can do this so it only affects the particular Rich Text Element that you want it to… and those with the same class name.
Images in RT are set with <figure><div><image></div><caption></figure>
You can’t style the div wrapping the image (that’s rare in webflow components), but you can style the figure element. With a class, or better, directly by selecting the figure element in the selector list lile you’d do with a paragraph, then limiting for the RT class.
You cans till target the unstylable div with custom code by using figure.class > div in CSS custom code.