Trying to show WordPress category query result as background image

The code is working fine so far, but I want to show the post thumbnail as a background image inside the li tag. Been trying many variations, but can’t work it out? Here’s the code:

<div class="main-slider" class="flexslider">
<ul class="slides">
    // Get all posts in 'slider' category
    $post_categories = wp_get_post_categories( $post_id );
    $args = array( 
        'post_type' => array('Interaction','Print','Motion','Image','Sound'),
        'numberposts' => 6,
        'category_name' => 'slider'
    $postQuery = get_posts($args);
    /*query_posts(array('post_type' => 'Interaction',
                        'post_type' => 'Print',
                        'post_type' => 'Motion',
                        'post_type' => 'Image',
                        'post_type' => 'Sound' 
                        )); */              
    foreach( $postQuery as $post ) : setup_postdata($post);
        if ( has_post_thumbnail() ) { ?>
            <li class="frontSlider"><?php the_post_thumbnail('feature-slider'); ?>
                <a href="<?php echo get_permalink(); ?>" title="Go to <?php echo the_title(); ?>" rel="bookmark">
                    <div class="flex-captionWrap">
                        <p class="flex-caption"><?php echo get_post(get_post_thumbnail_id())->post_excerpt; ?></p><!-- Retrieves text string from Captions field in Media -->
    endforeach; ?>
</ul><!-- .flexslider -->
</div><!-- .flexslider -->

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