[Plugin: Multiple Post Thumbnails] Images not working in Nivo Slider

I am trying to get this plugin to work with the Nivo Slider. In the functions.php I assigned the secondary feature image to be named slideshow-image.

Here is the original code for the Nivo Slider referencing the standard Feature Image.

<div id="slider" class="nivoSlider">
<?php while($featured_posts->have_posts()): $featured_posts->the_post(); ?>
<?php if(has_post_thumbnail()): ?>
<?php $image = wp_get_attachment_image_src(get_post_thumbnail_id($post->ID), 'feature-image'); ?>
<?php $image_thumb = wp_get_attachment_image_src(get_post_thumbnail_id($post->ID), 'featured-image-thumb'); ?>
<a href='<?php the_permalink(); ?>' title='<?php the_title(); ?>'><img src="<?php echo $image[0]; ?>" alt="<?php the_title(); ?>" title="#htmlcaption_<?php the_ID(); ?>" rel="<?php echo $image_thumb[0]; ?>" width='516' height='340' /></a>
<?php else: ?>
<a href='<?php the_permalink(); ?>' title='<?php the_title(); ?>'><img src="<?php bloginfo('template_directory'); ?>/timthumb.php?src=<?php bloginfo('template_directory'); ?>/images/thumbnail.png&w=516&h=340" alt="<?php the_title(); ?>" title="#htmlcaption_<?php the_ID(); ?>" rel="<?php bloginfo('template_directory'); ?>/timthumb.php?src=<?php bloginfo('template_directory'); ?>/images/thumbnail.png&w=70&h=60" width='516' height='340' /></a>
<?php endif; ?>
<?php endwhile; ?>
</div>

I tried putting the following code in but didn’t work.

<?php if (class_exists('MultiPostThumbnails')
    && MultiPostThumbnails::has_post_thumbnail('post', 'slideshow-image')) :
        MultiPostThumbnails::the_post_thumbnail('post', 'slideshow-image'); ?>

<?php $image = wp_get_attachment_image_src(get_post_thumbnail_id($post->ID), 'slideshow-image'); ?>

<?php $image_thumb = wp_get_attachment_image_src(get_post_thumbnail_id($post->ID), 'slideshow-image-thumb'); ?>

https://www.ads-software.com/extend/plugins/multiple-post-thumbnails/