Both editors included with moodle allow to add an image easily to the content (doesn't matter if it is the theme or a course page)
In my previous message I explained how to add an image to the marketing block. If you took a look to the Adaptable demo then you will see that you can add any kind of content to the Adaptable blocks (marketing and footer)
So it is really difficult to understand why it is not working for you.
Could you explain how do you use the editor file picker? Upload the image, use an internal repository, and external repository...?
What editor are you using?
Themes
Pictures not displaying properly in Themes (Adaptable)
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