Filter the media uploads baseurl to point to a CDN

cdn-baseurl.php

<?php

// Turn ON:  $ wp option update cdn_baseurl https://abc123.cloudfront.net/wp-content/uploads
// Turn OFF: $ wp option delete cdn_baseurl

$cdn_baseurl = get_option( 'cdn_baseurl' );

if ( ! $cdn_baseurl ) {

	return;

}

add_action( 'template_redirect', function () use ( $cdn_baseurl ) {

	$pattern = sprintf(
		'#https?://%s/(.+\.(?:gif|ico|jpg|jpeg|png))#',
		preg_replace( '#^https?://#', '', str_replace( '.', '\.', WP_CONTENT_URL . '/uploads' ) )
	);

	ob_start( function ( $content ) use ( $pattern, $cdn_baseurl ) {

		return preg_replace( $pattern, "{$cdn_baseurl}/$1", $content );

	} );

}, -PHP_INT_MAX );

add_filter( 'upload_dir', function ( $uploads ) use ( $cdn_baseurl ) {

	$uploads['baseurl'] = $cdn_baseurl;

	return $uploads;

}, -PHP_INT_MAX );