Full width images within post content

I believe you’ll want to set the image as a background image on the div (with inline CSS). In your template’s PHP you can use the following to get the URL to the full-sized featured image (within the Loop, of course):

$image_url = wp_get_attachment_url( get_post_thumbnail_id( $post->ID ) );

Then to assign this image as the background image, you’d do something like:

<div<?php if( $image_url ) { ?> style="background: url(<?php echo $image_url; ?>) no-repeat center center; background-size: cover;"<?php } ?>>