For every Leanpub book, you need to upload a cover image to Leanpub, in order to get the cover image to show up on your book's landing page and in our bookstore.
You can upload your book cover image here:
...making sure to replace YOUR_BOOK with your book's unique web address.
On that page you'll find some helpful information about image sizing and other details.
You can also find a link to this page on your book's "Overview" page:
For a brief overview of how to navigate around Leanpub as an author, please see this article.