I modified the plugin for my needs, by adding the thumbnails to the custom post type portfolio, by adding these lines:
line: 115 of post-list-featured-image.php
add_filter(‘manage_portfolio_posts_columns’, array(&$this, ‘posts_columns’), 5);
line: 118 of post-list-featured-image.php
add_action(‘manage_portfolio_posts_custom_column’, array(&$this, ‘posts_custom_columns’), 5, 2);
Adjust portfolio with the name of your custom post type and enjoy.
The custom post types where added to my site by a paid theme, so they had also overridden the columns that appear, but most users shouldn’t have that issue.
http://wordpress.org/plugins/post-list-featured-image/