Originally Posted by Faster
It would be more accurately stated as:
"After inserting the image, press ENTER to create a paragraph break between the image and the text. With the cursor on the right-hand side of the image use the Insert -> Chapter Break command from the menu bar (or press CTRL-ENTER)."
The reason for this is that (apparently) the chapter break is inserted at the end of the paragraph the cursor is in.
Actually, even your fix is not enough. I just tested it... I added several paragraph breaks, then added image at top. Then moved cursor into middle of excessive breaks, and split chapter.
And the cover page still 'stole' the header from the first file (in this case, my title page)--- so my title page had no header, and the cover had the header for the title page, which is what caused the issue to begin with (no first page, broken toc).
From what I can tell, if you add the cover to your first page then split, you will need to check your headers else have a broken epub.