I'm sorry, I'm probably going to expose my ignorance or lack of sophistication here, but I don't understand your question. I have uploaded several ebooks in which I took an image to use as the cover, and then overwrote the book title on that image. One of them is
Saladin and the Conquest of Jerusalem by Stanley Lane-Poole, in the ePub section at
https://www.mobileread.com/forums/sho...d.php?t=224264
The cover is from a painting by Gustave Doré, and I think could qualify as background image.
The html and CSS are certainly nothing fancy. Could this method not be used for an internal chapter? I presume you'll be starting the chapter on a new page.