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:
Going there will take you to the Book Cover page for your book (exactly what you see will depend on your writing mode and your Book Theme, and we do update this page from time to time, so it may not match exactly what you see here):
On this page, we provide some guidance about how to choose a cover image size here. The guidance is set automatically based on the Book Theme you have chosen for your book (for information about book Themes, please go here.)
To add a cover image, just scroll to the bottom of the page, click "Choose File" and select the image you want to use from your computer, and then click the "Update Book Cover" button.
After the process completes, you will now see the cover image at the bottom of the page, and on your book's landing page. Here's an example, with the middle of the page cut out:
If your book has never been published, uploading a book cover image here will update the image on the book landing page (and elsewhere on Leanpub) immediately.
If your book has ever been published, you will need to publish a new version of your book, to get the image to update on Leanpub.
Finally, in order to get the image to appear in the ebook itself, you need to create a new preview or publish a new version.
Here is what this book cover image looks like on Leanpub: