How to define a default post thumbnail

Paste the code below into your functions.php file. Don’t forget to update the code with your default image url on line 13.

add_action( 'save_post', 'wptuts_save_thumbnail' );

function wptuts_save_thumbnail( $post_id ) {

	// Get Thumbnail
	$post_thumbnail = get_post_meta( $post_id, $key = '_thumbnail_id', $single = true );

	// Verify that post is not a revision
	if ( !wp_is_post_revision( $post_id ) ) {
		// Check if Thumbnail exists
		if ( empty( $post_thumbnail ) ) {
			// Add thumbnail to post
			update_post_meta( $post_id, $meta_key = '_thumbnail_id', $meta_value = 'http://yoursite.com/your_image_url.jpg' );
		}
	}

}

Thanks to WPTuts for the snippet!