[Plugin: Nimble Portfolio] Thumbnails not showing up inside circle

I installed the plugin and it looks good if I could just get the thumbnails to show up. However, on my portfolio page, inside the circle for each item, there is only a white background. No thumbnail of the image is shown. How do I fix this?

I noticed that the background URL is blank for the element.

<div style="background: url( ) !important;" class="nimble-portfolio-item">
                                <a rel="lightbox[nimble_portfolio_gal]" href="http://mydomain.com/~test/wp-content/uploads/2012/06/Ad1.png">
                                    <div class="nimble-portfolio-rollerbg"></div>
                                </a>
                            </div>

When I move the mouse over one of the portfolio pieces, the ‘zoom’ appears and when I click on the item, the full version appears. These functions work good. How do I get thumbnails to appear though?

Thanks for the plugin. Looks great and I hope to get it working properly.

http://wordpress.org/extend/plugins/nimble-portfolio/