As explained in [GitBook's docs][1]. 1800x2360 px `cover.jpg` file at the root directory.
ImageMagick's convert command used (which produced 1574x2360 px covers):
"$ convert contrib/gfx/covers/tapa-EN-600dpi.png -resize 1800x2360 en/cover.jpg"
[1]: https://toolchain.gitbook.com/ebook.html#cover
