When thinking about the right size for images to fit on the page in your book, it's best not to worry about the image size (as they can be resized) but consider the image dimensions.
So, these are the width:height aspect ratios you'll want:
Portrait (single page) 2:3
Portrait (double page) 4:3
You can use a crop tool in an image editor before you upload to Book Creator and set the aspect ratio, and it can resize your images to fit. There is no cropping tool for images in Book Creator.
The largest image allowable by Book Creator is 2 million pixels. For best quality results prepare your image as large as possible. This ensures the best quality even after compression, or if the user zooms in. Book Creator will automatically shrink any images bigger than 2 million pixels to meet the limit.