Different thumbnail sizes WP

 Function:

if ( function_exists( 'add_theme_support' ) ) {
 add_theme_support( 'post-thumbnails' );
        set_post_thumbnail_size( 150, 150 ); // default Post Thumbnail dimensions
}

if ( function_exists( 'add_image_size' ) ) {
 add_image_size( 'review-thumbnail', 150, 200, true );
        add_image_size( 'movies-thumbnail', 400, 9999 );
}

Template code:

 <?php echo get_the_post_thumbnail($post->ID, 'medium'); ?>