In addition to the image recommendations in Interior Image Requirements, the following apply to working with images in fixed-layout books:
Images inside fixed-layout books must be in JPEG or PNG format. To reduce file size, use JPEG for any images that do not require transparency. Using JPEGs results in a smaller file size and better performance. We recommend using a quality of 85 for JPEGs.
Images inside fixed-layout books must not exceed 5.6 million pixels. You can calculate whether an image inside the book file exceeds 5.6 million pixels by multiplying the height and width of the image.