Wordpress Shortcode inside of plugin

Here is a snippet from the shortcodes.php file in one of the plugins I'm writing.

if($home_loop->have_posts()) {
        switch($display) {      

        case "static":

                while ($home_loop->have_posts()): $home_loop->the_post();

                        if(has_post_thumbnail() ) { 
                            $content .= '<div class="parallax">';
                            $content .= get_the_post_thumbnail($post->ID, 'wrapper-wide', array('class' => "img-responsive")); 
                            $content .= '</div>';
                            $content .= '<div class="container">';
                            $content .= get_the_content();
                            $content .= '</div>';

                            $content .= '<style>';
                            $content .= '.parallax{//css style here}';

                            $content .= '</style>;



                        } else {
                            //Something
                        }

                endwhile;

                break;

This works perfect. If I put a featured image thumbnail in my home post it will be displayed in my parallax div. The question I have is instead of placing the thumbnail inside of the div, I want to have it background-image: url(). I found this code below

<?php
$background = get_field( 'background_image' );

if($background):
?>
<style>
.main-wrapper {
 background-image: url(<?php echo $background ?>);
}
</style>
<?php endif; ?>

How would I incorporate and run something like this inside of the $content? The reason I'm trying to get the image as a background-image:url() is so I can add specific css styling like background-position, size, repeat, attachment.

via Chebli Mohamed