PHP: Using custom field instead of thumb id

I’m trying to change some theme short-code to pull thumbnail images from a custom field instead of a thumbnail id (because the custom field images are in place), and am getting super confused.

Anyone know how to do this?

global $FRONTEND_STRINGS;
        $count = 0;
        while (have_posts()) {
            the_post();
            $count++;
            $subtitle = get_post_meta(get_the_ID(), '_prime_page_subtitle', true);
            $thumb_id = get_post_meta(get_the_ID(), '_thumbnail_id', true);
            $preview_image_url = prime_attachment_image_url(array(
                                                                 'att_id' => $thumb_id,
                                                                 'width' => 206,
                                                                 'height' => 139
                                                            ));
            ?>

            <div class="<?php echo $column_class; ?> text-align-left <?php if ($count == $num_posts) echo 'last'; ?>">

                <?php if ($show_image && !empty($preview_image_url)) { ?>
                <img src="<?php echo $preview_image_url; ?>" width="100%" class="styled-image"/>
                <?php } ?>
                <h4><?php the_title(); ?></h4>

                <p><?php prime_recent_posts_excerpt(); ?></p>
                <a class="continue-link" href="<?php the_permalink(); ?>">
                    <?php echo $FRONTEND_STRINGS['continue_reading'];?></a>
            </div>
            <?php

        }
        ?>
        <div class="clear"></div>