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( ABSPATH . str_replace(get_home_url().'/', '', $image[0]) ) ) {
        return $image;
    }
    // Replace the image source with a placeholder at the proper dimensions.
    $image[0] = "http://placehold.it/{$image[1]}x{$image[2]}/FF6600/FFFFFF";
    return $image;
}
add_filter( 'wp_get_attachment_image_src', 'placehold_404' );

Breaks in WP 4.4 due to the addition of image srcset