I believe the generaic image is coming from the CMS Section.
in the admin, go to Contents, and edit the Home Page -> Contents.
when editing the theme, that screen just means that it sees changes that have been made but not saved, it allows yo to start form the unsaved point, or to discard the unsaved changes. For the most part you will usually chose to discard the unsaved changes.
This normally happens if you start the Design Editor, then close it without saving.