featured image is taking low resolution pic in wordpress code example

Example: adding custom featured image size in wordpress

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)