Wordpress filter to replace 404'd images with a placeholder, Good for when you're working locally but don't want to copy down the whole uploads directory.

placehold-404.php

<?php
/**
 * Dev Helper for 404'd Images
 */
function placehold_404( $image ) {
	if ( !file_exists( str_replace(get_home_url().'/', '', $image[0]) ) ) {
		// Replace the image source with a placeholder at the proper dimensions.
		$image[0] = "http://placehold.it/{$image[1]}x{$image[2]}/ffd602/000000";
	}
	return $image;
}
add_filter( 'wp_get_attachment_image_src', 'placehold_404' );