// without parameter -> Post Thumbnail (as set by theme using set_post_thumbnail_size())
get_the_post_thumbnail( $post_id );
get_the_post_thumbnail( $post_id, 'thumbnail' ); // Thumbnail (Note: different to Post Thumbnail)
get_the_post_thumbnail( $post_id, 'medium' ); // Medium resolution
get_the_post_thumbnail( $post_id, 'large' ); // Large resolution
get_the_post_thumbnail( $post_id, 'full' ); // Original resolution
get_the_post_thumbnail( $post_id, array( 100, 100) ); // Other resolutions