We recommend that for images in your book (as opposed to the cover image) you use images set to a resolution of 72 pixels per inch (PPI) and that you use the JPG format set to 80% compression. These settings should help keep the size of your book files smaller.
We provide detailed advice for each book on a page called "Image Sizing Help." You can find that page by going here::
leanpub.com/YOUR_BOOK/help/image_sizing
...making sure to replace YOUR_BOOK with your book's unique web address.
The advice here will change based on the size you choose for your book. You can find this page in your Book Tools menu if you click on Writing and then select Image Sizing Help .
The maximum cover image size for your book depends on the book "Theme" you have chosen. Here is an article about the book Themes::