Adiciona uma imagem padrão ao the_post_thumbnail

functions.php

// Adiciona uma imagem padrão ao the_post_thumbnail
function add_default_image_to_the_post_thumbnail( $html, $post_id, $post_thumbnail_id, $size, $attr ) {
	global $_wp_additional_image_sizes;

	if( ! $post_thumbnail_id ) {
		if( is_array( $_wp_additional_image_sizes[$size] ) ) {
			$sizes['width'] = $_wp_additional_image_sizes[$size]['width'];
			$sizes['height'] = $_wp_additional_image_sizes[$size]['height'];
		} else {
			$sizes['width'] = get_option( $size . '_size_w' );
			$sizes['height'] = get_option( $size . '_size_h' );
		}
		$html = sprintf( '<img src="http://placehold.it/%1$dx%2$d/F5F5F5/&text=Sem+Foto" width="%1$d" height="%2$d" />', $sizes['width'], $sizes['height'] );
	}

	return $html;
}
add_filter( 'post_thumbnail_html', 'add_default_image_to_the_post_thumbnail', 999, 5 );