How to define a default post thumbnail

add_filter( 'post_thumbnail_html', 'my_post_thumbnail_html' );

function my_post_thumbnail_html( $html ) {

	if ( empty( $html ) )
		$html = '<img src="' . trailingslashit( get_stylesheet_directory_uri() ) . 'images/default-thumbnail.png' . '" alt="" />';

	return $html;
}

Two simple ways to correctly define a default post thumbnail (i.e., featured image) in WordPress.

Source: How to define a default post thumbnail