the_post_thumbnail( $size , $attr)

/**
 * Display Post Thumbnail.
 *
 * @since 2.9.0
 *
 * @param int $size Optional. Image size. Defaults to 'post-thumbnail', which theme sets using set_post_thumbnail_size( $width, $height, $crop_flag );.
 * @param string|array $attr Optional. Query string or array of attributes.
 */
function the_post_thumbnail( $size = 'post-thumbnail', $attr = '' ) {
 echo get_the_post_thumbnail( null, $size, $attr );
}
get_the_post_thumbnail 函数声明
 * Retrieve Post Thumbnail.
 *
 * @since 2.9.0
 *
 * @param int $post_id Optional. Post ID.
 * @param string $size Optional. Image size. Defaults to 'post-thumbnail'.
 * @param string|array $attr Optional. Query string or array of attributes.
 */
function get_the_post_thumbnail( $post_id = null, $size = 'post-thumbnail', $attr = '' ) {
 $post_id = ( null === $post_id ) "Just In Time" filtering of all of wp_get_attachment_image()'s filters
 if ( in_the_loop() )
  update_post_thumbnail_cache();
 $html = wp_get_attachment_image( $post_thumbnail_id, $size, false, $attr );
 do_action( 'end_fetch_post_thumbnail_html', $post_id, $post_thumbnail_id, $size );
 } else {
 $html = '';
 }
 return apply_filters( 'post_thumbnail_html', $html, $post_id, $post_thumbnail_id, $size, $attr );

set_post_thumbnail_size( $width, $height, $crop)

set_post_thumbnail_size(100,0,true);

/**
 * Registers an image size for the post thumbnail
 *
 * @since 2.9.0
 */
function set_post_thumbnail_size( $width = 0, $height = 0, $crop = false ) {
 add_image_size( 'post-thumbnail', $width, $height, $crop );