two issues with the parameters in the get_the_post_thumbnail function
get_the_post_thumbnail($post_type, $thumb_id, $post_id = NULL, $size = 'post-thumbnail', $attr = '' , $link_to_original = false)
first: the default size is 'post-thumbnail'. i don't believe there IS a size called 'post-thumbnail'. the smallest, thumbnail size is simply called 'thumbnail'
second: the $post_id is null by default. in the example on the plugin page you pass this parameter as NULL, I suppose presumably b/c the $post_id will be automatically set to the current post by default. except this doesn't happen.
global $id; $post_id = (NULL === $post_id) ? $id : $post_id;
this $id variable has no value, so if you don't pass get_the_ID() in as the post ID parameter, you will not get an image.
seems like this should be:
global $post; $post_id = (NULL === $post_id) ? $post->ID : $post_id;