Ebook Formatting

I suggest that your starting point is to get the EPUB3 checker.
validator.idpf.org

This will detail any errors or problems.

This note from Apple may help you:

The book’s cover art that appears on Apple Books (also known as the marketing image) is the image delivered alongside the book asset and does not refer to the cover image included in the book asset. The pixel limit for the interior cover, as with all interior book images, is 4 million pixels.

The book’s cover art must use RGB color mode and should be at least 1400 pixels along the shorter axis. For best results, a good rule of thumb is to use an image that is a minimum of 300 dpi.

The book’s cover art file must be a high-quality JPEG with .jpg or .jpeg extension or PNG with .png extension.

The 4 million pixel limit on interior book images does not apply to the external cover/marketing image that is sent alongside the book asset.

It is important that you do not increase the size of a smaller image to meet the minimum image size dimension standard. Excessively blurry or pixelated images will be rejected.

Images (within the EPUB) cannot exceed 4 million pixels. Apple recommends providing images that are at least 1.5 times the intended viewing size up to a maximum of 4 million pixels. You can calculate whether an image inside the book file exceeds 4 million pixels by multiplying the height of the image with the width.

Images should be defined using img tags in the HTML. Styles such as dimensions and positioning should be defined in the CSS.

To ensure proper viewing of images in content, use the HTML img tag instead of wrapping images in svg:image.

For accessibility, the alt attribute for an image must be included; the value must be an appropriate replacement for the image.

Finally Apple recommends that you run final image assets through image optimizations, such as ImageOptim (which is a free, open source optimization tool for Mac users).