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' );

old file_exists() check was improperly checking a relative path. Prepending the ABSPATH fixes this check. I didn't notice at first since the site I was originally working on locally was an image heavy site that had had thousands of images. So it replaced all images without me noticing. This new check should actually check if file exists if you have a partial uploads directory.

Breaks in WP 4.4 due to the addition of image srcset