Navigate to Google.com and search for the title of your book.
Select "Images" at the top of the search results page.
Select an image for your book cover.
Click "View original image."
Right click on the image, select "Save Image as..." and save it to a location on your computer.
Connect your Kindle to your computer's USB port.
Open Calibre and click the Device icon in the Calibre toolbar.
Find the desired e-book on your Kindle. If it does not have a green check mark under the "In Library" column, you must add it to your Calibre library. Right click on the title and select "Add books to library."
Click the Library icon in the Calibre toolbar.
Right click on the e-book, select "Edit metadata" and then "Edit metadata individually."
Click "Browse" in the "Change cover" section of the Edit Metadata menu.
Select the image you downloaded.
Click Open, and click OK.
Right click on the e-book, select "Send to device" and then "Send to main memory."