I was talking about the size of image, not EPUB.
In Apple Books, there is also out-of-book Cover. It is not subjected to the 5.6 MN pixel limit and is required to be less than 20MB.
And I have another question.
9 years ago, I self-published a book and it is on Amazon. I want to add some to book details (Screenshot 1).
I contacted via Author Central (Screenshots 2, 3, 4).
I also posted a question on KDP Community (Screenshot 5).
No response yet.
Is all that I can do is to wait?
Hosun
Screenshot 1

Screenshot 2

Screenshot 3

Screenshot 4

Screenshot 5

I don't understand why the book portals ask for or allow such gigantic cover images. Seems like a waste of their storage space for images that will be displayed only at modest digital size. But not my call or concern, I guess. 🙂
I would simply update the materials for your book on KDP. There are supposedly rules and requirements for updates, and anything past some modest change is supposed to be set up as a new book or edition, but I have books I have updated at regular intervals and never been challenged on the point. I'd recommend not drawing too much attention, such as by asking KDP support specfic questions about a specific title.
KDP support is... nearly useless, even if you can get them to answer in the first place.
KDP's forum is... completely useless unless you ask a first-step, ignorant-novice question that can be answered with boilerplate or a pointer to a (probably outdated) KDP help page. I wouldn't bother trying to get useful responses or help from either entity.