// put all the category IDs of current post into an array
$categories = get_the_category( $post->ID );
$category_id_array = array();
foreach ( $categories as $category ) {
    $category_id_array[] = $category->cat_ID;
}

// define how many thumbnails to show (at most)
$thumbnail_max = 10;

// grab all posts in the same categories as the current post
$category_thumb_args = array( 'category__in' => $category_id_array, 'posts_per_page => $thumbnail_max', 'exclude' => $post->ID );
$category_thumb_query = new WP_Query( $category_thumb_args );

// if there are posts loop through them
if ( $category_thumb_query->have_posts() ) :
    while($category_thumb_query->have_posts()) : $category_thumb_query->the_post();
        // if the post has a post thumbnail display it
        if ( has_post_thumbnail() ) :
            the_post_thumbnail( 'thumbnail' );
        endif;
    endwhile;
endif;

// reset the query since we used the_post()
wp_reset_query();