Set a default thumbnail

default-thumbail.php

<?php
// To work, this function require to bypass has_post_thumbnail() test into templates…
add_filter( 'post_thumbnail_html', 'default_post_thumbnail_html', 10, 5 );
function default_post_thumbnail_html( $html, $post_ID, $post_thumbnail_id, $size, $attr ) {
	// If no thumbnail is sendback by get_post_thumbail(), return a default image
	if ( '' == $html && in_array( get_post_type( $post_ID ), array( 'post', 'page' ) ) ) {
		// Get a default thumbnail ID into a list
		$thumbs = get_option( 'options_default_thumbnails', array() );
		$thumb = $thumbs[ array_rand( $thumbs ) ];
		remove_filter( 'post_thumbnail_html', 'default_post_thumbnail_html', 10, 5 );
		$html = wp_get_attachment_image( $thumb, $size, '', $attr );
		add_filter( 'post_thumbnail_html', 'default_post_thumbnail_html', 10, 5 );
	}
	return $html;
}