Display all posts thumbnails when viewing a single post

You would have to get all the posts but the current one with a wp_query then loop through them and output their thumbnail.

Something like this (Should work but untested)

<?php
//Set the current page id
$current_id = get_the_id();

// Get all posts of a post type but the current post
$the_query = new WP_Query( array(
    'post_type' => 'your-post-type',
    //'post__not_in' => array( get_the_id() ), Removed to include current post
    'posts_per_page'=>-1
) );

// The Loop
if ( $the_query->have_posts() ) {
    echo '<ul>';
    while ( $the_query->have_posts() ) {
        $the_query->the_post();
        if( $current_id  == get_the_id() ){
            $current="active";
        } else {
            $current="";
        }
        echo '<a class="'.$current.'" href="'.get_permalink( get_the_id() ).'">';
        echo get_the_post_thumbnail( get_the_id() );
        echo '</a>';
    }
    echo '</ul>';
} else {
    // no posts found
}

// Restore original Post Data
wp_reset_postdata();
?>

EDIT: added ‘posts_per_page’=>-1 to the query args

EDIT2: Show the current post with a class of active