If you make the chapter number element large enough you can apply a background image.
Here's an example of adding an image to every page but it can easily be adapted to specific elements like chapter number elements.
https://ebooks.stackexchange.com/que...-or-epub-books
Also a quick Google found -
https://www.mobileread.com/forums/sh...d.php?t=227739 which is exactly what you are asking.