Replace all broken attachment images with a default image - great for doing local dev with an imported database

image-replacer.php

add_filter( 'wp_get_attachment_image_attributes', function( $attr, $attachment ) {

    $content_path = ( defined( 'IMAGE_REPLACER_WP_CONTENT_PATH' ) ) ? IMAGE_REPLACER_WP_CONTENT_PATH : WP_CONTENT_DIR;

	$content_url = ( defined( 'IMAGE_REPLACER_WP_CONTENT_URL' ) ) ? IMAGE_REPLACER_WP_CONTENT_URL : WP_CONTENT_URL;

	$file_path = str_replace( $content_url, $content_path, $attr['src'] );

	if ( file_exists( $file_path ) )
		return $attr;

	$attr['src'] = ( defined ( 'IMAGE_REPLACER_DEFAULT_IMAGE_URL' ) ) ? IMAGE_REPLACER_DEFAULT_IMAGE_URL : $content_url . '/default.png';

	return $attr;

}, 10, 2 );