php within html within php

You can format it as, there’s also no need to put the echo

<?php next_post_link('%link',"<img src="https://wordpress.stackexchange.com/questions/251458/. CFS()->get("image_location') . "/>"); ?>

CFS()->get('image_location') gets the field based on the current post id. If you want to use it for the next or previous, you have to pass the next or previous posts ID.

You can do something like this:

$next_post = get_adjacent_post( true, '', false, 'taxonomy_slug' );
$next_post_id = '';
if ( is_a( $next_post, 'WP_Post' ) )
    $next_post_id = $next_post->ID;
if ( ! empty( $next_post_id ) ) {
    $next_post_image_source = CFS()->get( 'image_location', $next_post_id );
} else {
    $next_post_image_source = "default image";
}

next_post_link('%link',"<img src="https://wordpress.stackexchange.com/questions/251458/. $next_post_image_source ."/>"); 

No assurance this would work but it should be easy to continue from here