To find helpful guidance for sizing images in your book, go to the "Image Sizing Help" page for your book here:
https://leanpub.com/YOUR_BOOK/image_sizing_help
...making sure to replace YOUR_BOOK with your book's unique web address.
You can also find the "Image Sizing Help" page on your book's "Overview" page:
There are two concepts:
Resolution, which is pixels per inch (PPI) aka dots per inch (DPI).
Size, which is how many actual pixels your image is, in length and width.
What you should do is set a resolution of your images such that, when you divide the size by that resolution, they fit on a printed page, respecting margins.
Example:
If I have an image which is 900 pixels wide, then if its resolution is 300 PPI, it will look like 3 inches (900 / 300) wide. This will fit in almost all of our books, based on the size chosen. HOWEVER, if its resolution is 72 PPI, then it will look like 12.5 inches wide (900 / 72), which will fit in no books we produce.
So, the point is that the width of an image in pixels and its resolution in pixels per inch produce a size in inches. So, the same size image (in pixels) may or may not fit.
Now, when we encounter an image which does not fit, we do things. Hence all the warnings...
[Note: If you're not sure how to change the resolution of an image, this article might help: How do I change the resolution of an image?]