Get post thumbnail by post id ("thumbnail,medium,large,full") in wordpress.

Get post thumbnail by post id ("thumbnail,medium,large,full").

<?php
// without parameter -> Post Thumbnail (as set by theme using set_post_thumbnail_size())
$postId=12;
get_the_post_thumbnail($postId );                 

get_the_post_thumbnail( $postId, 'thumbnail' );      // Thumbnail (Note: different to Post Thumbnail)
get_the_post_thumbnail( $postId, 'medium' );         // Medium resolution
get_the_post_thumbnail( $postId, 'large' );          // Large resolution
get_the_post_thumbnail( $postId, 'full' );           // Original resolution

get_the_post_thumbnail( $postId, array( 100, 100) ); // Other resolutions

You can also give Post Thumbnails their own class:
// Give the Post Thumbnail a class "alignleft".
echo get_the_post_thumbnail( $post_id, 'thumbnail', array( 'class' => 'alignleft' ) );