The cover page outside the cover
A book's cover is an extremely important element in marketing the book, in displaying it in bookstores, and in the marketing efforts that accompany its sale. In the move to a digital format, the cover is no less important - it's what appears on the user's library screen, and it's what has to remind them at a glance which book this is, even if the cover image is shown on its own, without the title or author's name beside it. Defining the cover image in the EPUB3 format is also a task worth giving some thought to. A cover image at too high a resolution will burden the reading app and the user's device. A cover image at too low a resolution will look bad on an 8-inch tablet. And in general - where and how do you actually define a cover image?
The recommended ratio for a cover image is 1:1.5, and a resolution of 400:600 is more than enough. When using the "Sigil" tool to build books, you can, with a single click, link an image and designate it as the cover image. "Sigil" will automatically insert the appropriate and valid (almost) settings into the OPF file, but on top of that it will create a new file - a new "chapter," in fact - that consists entirely of a single page displaying nothing but the cover. Most books converted here in Israel are set up this way, with a first page that shows only the cover. Why? Just because. In practice, there is no real need for this page, and sometimes the habit needlessly slows down opening the book, when the first page is only the cover image - the very image the user saw a second before they opened the book.
So the recommendation is to let "Sigil" do its job in designating the image you chose as the cover image (a link to a valid identifier for the image object in the OPF file), but once that's done, it's best to simply delete the new COVER.XHTML file from the chapter list. And one more tip - for compatibility with other HTML standards - it's a good idea to replace the image identifier that "Sigil" assigned - usually it's just the file name itself - with a different identifier, without a period or spaces, of course.