Section 29.12 Cover Images
The docinfo/covers element may be used to specify the filenames (with relative paths) of a front-cover image and a back-cover image via the @front and @back attributes. The image must be a single-page PDF, and it will be scaled to fit an entire page. So it is your responsibility to supply an image which has the correct aspect ratio and sufficient resolution. These are only supported for the LaTeX conversion.
This is meant to help you create a professional electronic PDF. A print-on-demand service (Chapter 41) will likely want a standalone image (possibly with the front and back, plus a spine, all rolled into one wrap image). So build your real cover images (Chapter 40), and then modify them for this use.