Retrieve the post thumbnail.
get_the_post_thumbnail( int|WP_Post $post = null, string|int[] $size = 'post-thumbnail', string|array $attr = '' )
Example: show the feature image
<?php echo get_the_post_thumbnail(get_the_ID()); ?>
Example: show the feature image under different sizes.
// 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