Excellent question there. I would do this. Try a plugin called Visual Term Description Editor (https://wordpress.org/plugins/visual-term-description-editor/). What it does is replace the default text only box for menu group descriptions with the same editor you have on the edit page/post screen. That will allow you to add in images along with any descriptions of that menu group. I haven’t tested it, but that would appear to give you the ability to add in an image above each group.
If you have any questions or run into trouble, let me know.