Problem with calling thumbnail image

I’m using a theme from Theme Forest called Immersion:
https://themeforest.net/item/immersion-responsive-fullscreen-wp-theme/2346010

I’m using a portfolio plugin called Nimble:
https://themeforest.net/item/immersion-responsive-fullscreen-wp-theme/2346010

In the portfolio plugin, everything seems to be working except one thing. The thumbnails on the front-end are not showing up. I am trying to fix the problem myself but not sure what to do. The plugin is quite simple and only has a few files, so I thought it might be an easy fix.

Can anyone here help me out?

Here is the code from template.php

<div class="nimble-portfolio-holder">
                            <?php $src = wp_get_attachment_image_src(get_post_thumbnail_id($post->ID), array(303, 203), false, ''); ?>
                            <div class="nimble-portfolio-item" style="background: url(<?php echo $src[0]; ?>) !important;">
                                <a href="<?php echo nimble_portfolio_get_meta('nimble-portfolio'); ?>" rel="lightbox[nimble_portfolio_gal]" >
                                    <div class="nimble-portfolio-rollerbg"></div>
                                </a>
                            </div>
                            <div class="nimble-portfolio-title">
                                <a href="<?php the_permalink(); ?>" class="button-fixed">
                                    <?php _e('Read More →', 'framework') ?>
                                </a> <a href="<?php echo nimble_portfolio_get_meta('nimble-portfolio-url'); ?>" class="button-fixed">
                                    <?php _e('View Project →', 'framework') ?>
                                </a>
                            </div>

and here is the output code from browser. You’ll notice the background URL () is blank for some reason and I’m not sure why?

<div class="nimble-portfolio-holder">
                                                        <div style="background: url( ) !important;" class="nimble-portfolio-item">
                                <a rel="lightbox[nimble_portfolio_gal]" href="https://66.147.255.218/~test/wp-content/uploads/2012/06/Ad1.png">
                                    <div class="nimble-portfolio-rollerbg"></div>
                                </a>
                            </div>
                            <div class="nimble-portfolio-title">
                                <a class="button-fixed" href="https://66.147.255.218/~test/wp-content/portfolio/Ad1/">
                                    Read More →                                </a> <a class="button-fixed" href="https://66.147.255.218/~test/wp-content/uploads/2012/06/Ad1.png">
                                    View Project →                                </a>
                            </div>
                        </div>