The PreTeXt Guide

The publication file may be used to specify the filenames of a front-cover image and a back-cover image. 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. See Subsection 44.3.7 for exact syntax. Implementation is via the LaTeX pdfpages package.

This is meant to help you create a professional electronic PDF. A print-on-demand service (Chapter 43) 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 first (Chapter 42), and then modify them for this use.