You don't have to renumber. Name the new file anything you want (as long as the name's not already taken), then just drag that new, blank file to the top of list (drag 'n' drop) and insert your image (make sure that the image shows up). You can verify that the file's marked with the "Cover Image" semantic, but current versions of Sigil will automatically tag it as "Cover Image" if it's first, and contains one and only one image.
You may need to do some styling to center the image and size it properly, but that's beyond my basic online tutorial skills to describe.
