The classnames for Image blocks can't be changed, they are fixed to:

caption-style-medium caption-alignment-center

We enforced those classes to make the native Image block match our styles, but in the way it's implemented the values can't be changed.

See:
https://github.com/greenpeace/planet4-plugin-gutenberg-blocks/blob/master/assets/src/ImageBlockExtension.js#L80-L88

This is important as the users can't avoid images from triggering a Lightbox using the class `force-no-lightbox`, which in some cases is needed for decorative images, logos, image headers, etc.