You can upload a book cover by going here:
...making sure to replace YOUR_BOOK with your book's unique address.
You can also find a link to the "Book Cover" page under "Settings" on the Overview page for any Leanpub book:
Here is what the Book Cover page will look like the first time you see it:
We provide some guidance about how to choose a cover image size here. The guidance is set automatically based on the Theme you have chosen for your book (for information about book Themes, please go here.)
To add a cover image, just click "Choose File" and select the image you want to use from your computer, and then click the "Update Book Cover" button. You will now see the cover image there, and on your book's landing page:
In order to get the image to appear in the book itself, you need to create a new preview or publish a new version.