I would say those 3 are features that are not core to a CMS. Some CMS are headless, hence don’t need themes. If by WYSIWYG you mean drag and drop page builder, there were CMS before this exists. Images is just a type of content usually called Assets. Most CMS include a Digital Asset Management module, but, again, it’s an added feature.
I would venture the minimum set of features of a CMS would be:
- creation and editing of content
- a way to organize of content ( categories, tags, content types )
- user management
- content publishing ( publish, unpublish )
- content storage ( db )
then, there need to be a way to distribute that content, depending on the platform. On the web, it would be linking a URL to a piece of content.
