Calling array in shortcode issues - php

Question

I am condensing some area of duplicate code over multiple template into one shortcode that I can reuse easily.
I am calling the featured image url as an inline background image style at it works great! Here's the code.
<?php $image = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), 'single-post-thumbnail' ); ?>
<?php while (have_posts()) : the_post(); ?>
<div class="single-casestudy" style="background-image: url('<?php echo $image[0]; ?>'); background-size: 100%;">
<div class="case-study-content">
<h4><?php the_title(); ?></h4>
<p><?php echo excerpt(22); ?></p>
</div>
</div>
<?php endwhile ; ?>
This works fine when not in the shortcode, and I can see my featured image as the background on the live site, however when putting it into a shortcode function, it doesn't error, it just doesn't work! Heres the code I am using for the shortcode functionality. It seems like the $image[0] is not pulling through any data into the array.
function otherinfo_function() {
$args = array( 'post_type' => 'casestudy',
'posts_per_page' => -1,
'orderby' => menu_order,
'order' => RAND
);
query_posts($args);
$image = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ),'single-post-thumbnail' );
while (have_posts()) : the_post();
echo $image[0];
$output = $output . '<div class="single-casestudy" style="background-image: url(' . $image[0] .'); background-size: 100%;">';
$output = $output . '<div class="case-study-content">';
$output = $output . '<h4>' . get_the_title() . '</h4>';
$output = $output . '<p>' . excerpt(22) . '</p>';
$output = $output . '</div>';
$output = $output . '</div>';
endwhile ;
$output = $output . '</div>';
return $output;
}
add_shortcode('otherinfo', 'otherinfo_function');
If anyone could help that would super great!
Sam