Click the image to open in a new browser window or tab, then right-click the image and select Save Image As….
To update your Facebook Cover image, hover your cursor over the cover, and click the Change Cover button
Select Upload Photo… from the menu.
Navigate to the new cover you want to upload, select it and click OK. Facebook will then let you re-adjust the positioning of your cover, although the covers below are generally “ready for use” without having to do much repositioning.