A cover is a requirement of fixed layout eBook retailers. This is the image shown when the book is on the ‘bookshelf’ in the store and for the reader.
Using first page as cover
By default CircularFLO will ask whether you want to use the first page of your book as the cover image or to add another JPEG. If choosing a JPEG we recommend an RGB JPEG with a pixel size of between 1000 and 1600 pixels on the longest edge.
There is a Preference option to use the first page as the cover image without prompting.
Covers for Audiobooks