Need thumbnail image from custom fields

Here’s the link to the page: https://www.jamandcheese.be/wordpress/2011/05/huisstijl-dierenarts-jeffrey-poppe/

I want to display a list of thumbnails in the sidebar, these thumbnails are stored in a custom field.

This is my code for the widget:

<?php $my_query = new WP_Query('category_name=drukwerk'); ?>

<?php while ($my_query->have_posts()) : $my_query->the_post(); ?>

<a class="side-thumb" href="<?php the_permalink() ?>" title="<?php the_title(); ?>"><img src="<?php echo get_post_meta($post->ID, 'img_thumb_list', true) ?>" alt="<?php the_title(); ?>" title="<?php the_title(); ?>"/></a>

<?php endwhile; ?>

So I use ?php echo get_post_meta($post->ID, 'img_thumb_list', true) ?> to get the image in the custom fields, but it doesn’t work. If you check the generated code it shows the src=”” is empty!? What’s going wrong here?

PS: I’m using this plugin PHP Code Widget.

[No bumping, please.]