Do you want to add the featured image as background image to Beaver Themer Post Navigation module? You are creating the single post details page layout with Beaver Themer Layout and using the Post Navigation module for prev/next post nav. By default you will get this kind of design for post navigation.
In this article I’m showing how you will get this kind of design (like attached screenshot).
Adding New Image Size: First adding the new image size for post nav. Open the functions.php file of your active theme and add the following PHP snippet. Getting the more details about add_image_size function from here.
I am creating the 800 x 220 px image. You will adjust the image size based on your site design.
add_image_size( 'post-nav-thumb', 800, 220, true );