Try This

the_post_thumbnail(); // Without parameter ->; Thumbnail
the_post_thumbnail( \'thumbnail\' ); // Thumbnail (default 150px x 150px max)
the_post_thumbnail( \'medium\' ); // Medium resolution (default 300px x 300px max)
the_post_thumbnail( \'medium_large\' ); // Medium-large resolution (default 768px x no height limit max)
the_post_thumbnail( \'large\' ); // Large resolution (default 1024px x 1024px max)
the_post_thumbnail( \'full\' ); // Original image resolution (unmodified)
the_post_thumbnail( array( 100, 100 ) ); // Other resolutions (height, width)